From what I’ve read, both seem acceptable. But what about in a professional context? Is either preferable?
I’ve got to admit, the single-word “firsthand” just bugs me for some reason, but I want to go with what’s considered proper. Also, if two, should it be joined by a hyphen?
As more or less synonymous to “eyewitness” and antithetic to “hearsay”, it’s a single word: firsthand. I suspect but don’t know for sure that it was coined to parallel “secondhand.”
There are usages where it is properly two words, e.g., the person in a card game sitting at dealer’s left and therefore being the first hand to play, or the first hand to be dealt of a new game. Or “after grasping the handle firmly with one hand, take the other hand and twist the control firmly counterclockwise. Then move your first hand up the handle…”
The GPO Style Manual (pdf of the applicable section), usually the best source for this sort of thing, says it’s one word.
Such words move in a progression from:
There’s usually a long period where they are changing. Which form is ‘correct’ depends on the context, the formality of the writing, and the age of the writer, among other things.
If you look at books from the 1700’s and 1800’s, you will see “to-day” and “to-morrow” frequently. Those terms were in the second stage of that progression at the time. But most modern readers don’t even realize that these terms started out as 2 separate words.
I should say that the GPO Style Manual does lean toward writing terms solid (no hyphen or space).
I have many books from the 1920s and 1930s, especially British books, that continue to use the hyphen in those words.
I tend to use words/phrases like this as two words when an adverb (“I found out about this first hand”), and as one word when an adjective (“I have firsthand knowledge of this”) – not sure if that’s common, or considered “correct”.