Where does the word "trivial" come from?

Solution 1:

OED to the rescue again...

Etymology: < Latin triviālis, in sense A.5 below, < trivium (see trivium n.); compare French trivial (16th cent. in Godefroy Compl.).

So yes, it comes ultimately from trivium = "three ways", but A.5 says

5. Such as may be met with anywhere; common, commonplace, ordinary, everyday, familiar, trite. Now rare (passing into A.6).

6. a. Of small account, little esteemed, paltry, poor; trifling, inconsiderable, unimportant, slight.

Trivial comes from the Latin trivialis which means very much what trivial does. Quite how the Latin trivialis was derived from trivium is beyond the scope of an English site, but it may well be to do with idle chatter on a street corner.