It always strikes me as odd when an adjective that ends in y doesn't have a dictionary defined root noun (funny≈fun; angry≈anger; silly≠sill; etc). More specifically, I'm trying to write a lyric, and a noun form of surly [intolerant/unpleasant/condescending], with the rhyme scheme keeping in "url" sound (intent to loosely rhyme with "world"), again, like the would-be word "surl" would be perfect. I know that in music you can get away with some grammatical/linguistic finagling, but as it seems nonexistent in dictionaries, I'm hesitant to use it. In anyone's opinion, is it understandable/acceptable, for use in either a lyric or sentence?
Surly is not surl+y, it is from sur+ly, an alternate adjectival ending prominent after words ending in an r in English, like burly, brotherly, etc. It may derive from English's tendency to add an additional sound to create a digraph after the 'r' sound, for reasons dating back to middle English, and probably consciously overlapping with the -ly adverbial use.
Plenty of words end in the -irl/-url sound in English without create false back-formations, in my opinion.
The isn't any root surl here. As the comments have said, surly is a variation of the now obsolete word sirly, which itself is from sir + -ly. The earliest attestation for surly in the Oxford English Dictionary (OED) is in A medicinable morall (1566).
I don't see any evidence of surl being used as a word at all (although it's easy to find it e.g. as an acronym with a completely different meaning: SURL). I checked OED, COCA, and also did some Google searches (both regular and in Google Books).
This doesn't, however, mean that it wouldn't be understood in your poem, although it's hard to say for sure without context. Because the etymology of surly is opaque (and thus few people will be aware of the word's real etymology), surl seems like it could be a real word derived from surly.