I’ve a Firefox add-on where a mouse-over and an alt-click takes me directly to a definition. Even my own words here.
I like this idea and will start doing the same instead of looking words up at the moment.
My Kindle seems to be no match for Cormac McCarthy’s Blood Meridian.
The last word it couldn’t define was catclaw, which I think is a type of tree. Instead of saying they “walked among the trees” McCarthy will always tell you the name of the tree, e.g., “walked among the catclaw and crucifixion thorn…”.
If I’m reading on my Nook, I even look up words I know. Sometimes it will be a word I know, but don’t feel like I could define if asked. Sometimes it will be one that I don’t think the author is using correctly. I looked up all kinds of words while reading the A Song of Ice and Fire books because I wasn’t always sure whether they were real words or made up for the books.
It happens that the word concerned, is one which I know; but if I’d first come across it in the above quotation, initial thought would have been: “The poor quondams – what did they ever do to him?”
Most of my reading is for recreation and entertainment; at the risk of coming across as boastful, I seldom encounter a word that I don’t know. When I do, I sometimes look it up; or sometimes try to figure it out from context, or just let it go by – depends on mood.
Likewise.
It isn’t very often that I come across a word that I don’t know, but if I do I immediately look it up, and did even before the internet when it involved grabbing a dictionary.
I remember many years ago Australian comedy legend Barry Humphries used to write a column in one of the Sunday papers. They were very funny but he made a habit of using, once or twice, in every column some obscure word when a perfectly obvious alternative would have done. I longed to read a column without resorting to the dictionary but never achieved it.
I consider myself a fairly accomplished reader with a respectable vocabulary, most of the time I can deduce the meaning of a word from the context in which it is used. Since the majority of my reading is done on the computer, Google is only a ‘click away’. But I still keep a Webster’s New World dictionary and thesaurus handy. 
The word ‘otiose’ threw me, though. 
And I never would have guessed that it’s pronounced o-she-os, if I hadn’t looked it up. 
Sometimes I’ll be writing an email or a Straight Dope post, and I’ll want to use a word that sounds right, but I’m not 100% sure what it means. So I’ll look it up just so to keep me from looking like a giant ass.
Sometimes I’ll find myself saying a word when I’m speaking to someone, and I’ll double-check to see if I used the word correctly. They aren’t always GRE words.
Every word if I’m reading a novel. It builds my vocabulary. I have a dictionary next to my bed where I read. Then I bought a Kindle, and it has a dictionary built in so you just have to click on a word to get the definition. I read a few books by Neal Stephenson (Snow Crash, Cryptonomicon, Reamde) and that guys always sends me to the dictionary.
When doing other types of reading it doesn’t happen as often (magazines and newspapers don’t use as broad a vocabulary) and I don’t always take the time because it’s not as convenient.
Anthony Burgess was another one who could only be read without a dictionary by the same people who do crossword puzzles with pens. I’d just finished reading everything he had out when he died 20 years ago last month. Simply mourning him wasn’t adequately obscure: better to express it as a threnody.
I haven’t read Time “backward ran sentences until reeled the mind” Magazine in years, but vocabulary smartassery was long their pride. I recall headlines like “Sounding the Tocscins for Toxins.”
I’m another diligent definition seeker. I use an e-book reader nowadays, so it’s even easier. I’ve been reading books from Project Gutenberg mostly, so most of the words I look up have to do with things like fashion or fortifications in Ye Olde Days.
I don’t encounter unfamiliar words all that often, and I usually pick up enough from context to carry on with. If I don’t, or if what I’m reading is not particularly absorbing, I’ll stop to look it up. Otherwise, I’ll look it up later. Either way, I check it eventually.
I just did. (Though it was more from roots than context. Is it cheating that I already knew what baryons are?) Looking it up confirms that it is what I inferred.
Another of that breed is Lawrence Norfolk: author of The Pope’s Rhinoceros, In the Shape of a Boar, and a couple of other novels. My brother, whose tastes in fiction are more highbrow than mine, is a Norfolk fan; says that when he reads a book by this guy, he always needs a dictionary close at hand. (People’s mileages vary: on brother’s recommendation, I tried a Lawrence Norfolk novel, but couldn’t get far with it – struck me as pretentious, precious, and basically dreary.)
My brother’s all-time favourite “Norfolk word”, is “lemniscate”: signifying “adorned with ribbons”, and it also has a meaning in geometry.
Every day.
And, based on this thread, the rest of the afternoon, too. 
Ever since I bought my Kindle…ALL THE TIME!
It has become quite quotidian.
Like a lot of people have already said. My Kindle has made this the easiest thing in the world to do.
The other most used feature in my Kindle is the word search feature. Don’t recognize a characters name? Boom! You tap the word, and search the book for it, and you get a listing of that word and several words before and after it, wherever it’s used in the book. Now when I come across a person’s name that I don’t remember who they are it’s easy to refresh my memory.
Because of those two features I’m a Kindle fan over paper copies.
I find that a much more expurgatory method is to misuse the word on the internet. Someone will soon be along with the definition.
Almost always, especially if I’m near my iPad. I love words. I keep a list of my favorite words, and I’m surprised that I continue to come across words I don’t already know that aren’t technical or ancient.
All the time.
If I’m reading something on the web, and come across “louche” for instance — right click, look up, and boom, a pop-up bubble with the definition.
Composing a post or email, I’ll correct typos that are red-underlined, but the right-click/look-up thing, very handy if I’m unsure that’s the word I want.
On my iPhone or iPad, same thing, especially if reading a Kindle or iBooks book. Tap a word and the definition’s right there.
Other times I’ll just keep my Dictionary app up or at the ready if I need a word check or a thesaurus.
I did not read all of the replies yet, maybe this weekend.
I often look up words that I do not know. I will also look up words that I think that I know, especially if it has been used in what I consider to be an odd way.
It is all about fighting ignorance. The internet has made looking up a word easy, what with cut & paste and all!
Just five minutes age I looked up probity. It was used in a thread here about whether I trust LEOs or not.
I will let the reader choose whether they want to look probity up or not. Do you choose to fight ignorance or not? Heck, you probably already know what it means.