Back in the days, I always had my faithful Langenscheidt’s German-English/English-German dictionary at hand when I was reading an English book. Now I just look up unknown words in the Oxford dictionary on my Kindle, which is so convenient. Just like @Elmer_J.Fudd, I read a lot of stuff about pop music, but I don’t immediately listen to songs mentioned that sound interesting to me, but make mental notes and listen to them later on my living room stereo (I do most of my reading in bed).
I constantly look things up when I’m reading or watching TV. It expands both my general knowledge and vocabulary.
A good example is propinquity, a word I had never heard of before I encountered it in one of the Poldark novels:
Come on now, how many of you would have known what it means if you hadn’t been told? 
I knew it from this Michael Nesmith song.
Me. It’s a useful word to describe certain situations.
Propensity, I knew. Propinquity, I hadn’t a clue. I can usually figure out meanings from context, but in this case it wasn’t easy.
Constantly. My mind is like a magpie. This is why I sit down to read/surf, and when I look up it’s dark out and I’ve accomplished nothing all day.
Yes, definitely. Sometimes I’ll be reading something without an electronic device handy and I’ll try to keep my question in mind until I reach a place where I can use Google/Wikipedia.
Oh yes, all the time. If it’s an old book with a lot of cultural references unknown to me, I’ll keep a pad of paper collecting references with page numbers, to look up later. I recently finished Brideshead Revisited for the first time (a masterpiece!); here is a sampling of my noted things to look up from the first 45 pages:
- In the barracks: a broken window attributed to either wind or to “Sappers’-demonstration”
- Of someone who had not had the right sort of education: “He had not as a child ridden with Rupert’s horse”
- “Oxford - submerged now and obliterated, irrecoverable as Lyonnesse, so quickly have the waters come flooding in”
- “we were among dry-stone walls and ashlar houses”
- “he ate a very heavy meal of honey-buns, anchovy toast, and Fuller’s walnut cake”
- “I displayed … most painful to recall, a porcelain figure of Polly Peachum”
- “I put myself unreservedly in the hands of Dolbear and Goodall” [one assumes: a headache powder]
- “All b-boatmen are Grace Darlings to me.”
- “iced black coffee and charcoal biscuits”
We come for either one or two weeks. Just one week this year. Elmore Leonard is easy to read and I guess I read fast. I’m 3/4 of the way through it and will finish it in bed tonight.
All the time. It’s why I’m chock full of interesting and completely useless information.
There was a chemist’s shop in Oxford called Dolbear & Goodall.
I sometimes look things up, but not often. Sometimes while doing something else I’ll learn something about a book I’ve read.
Harry Turtledove has a series of novels about an empire called Videssos. The setting is pretty much like the Byzantine Empire, of which Turtledove was a scholar. The orthodox religion of Videssos is a duality, and the good god is named Phos, which I knew meant light. But while studying Koine Greek I learned the work for darkness, skotos, and that’s the name of the evil god.
On Kindle, it’s easy: you just put your finger on a word you don’t know and hold it for a second, then the definition pops up. Don’t need it often, but it’s sure handy.
Propinquity is one of those words I’ve looked up a dozen times yet have only the vaguest recollection of what it means the next time I run across it. Maybe if I listen to the Michael nesmith song I’ll remember it next time.
On occasion, usually to look up words I don’t know. For example that’s how I found out that “catamount” means what I’d call a mountain lion or cougar, since Lois McMaster Bujold used the term in one of her Sharing Knife novels. I’d run across it a few time before but just assumed it was the name for some fantasy critter like a manticore.
“Quondam” is another of hers I had to look up, when the protagonist called another character a “quondam murder” in Paladin of Souls."
For reasons which I do not understand, but I still find amusing , my wonderful English teacher in high school would say the phrase “quondam, erstwhile, whilom” from time to time, apropos of basically nothing, and so all of those stuck in my head.
Not until I started reading e-books on my tablet. While I look up definitions periodically, what I really like is that I can highlight something and look it up online. Since I read a lot of urban fantasy romance, I am always checking to see if this creature is based on real folklore.
In Robert Massie’s joint biography, Nicholas and Alexandra. there is a story told by the princesses tutor. He told them if they didn’t understand a word(they were learning French) to mark it down and ask him. One girl, I think Marie, but not sure, couldn’t find the tutor so she asked her father what “merde” meant. The tutor was embarrassed but the Tsar was quite understanding, realizing what had happened.
The most recent thing I looked up was the word “mieskeit” (from the closed captioning in the movie “Love and Death”).
Constantly.