This really seems to be a word that changes with the generations. Its superficial resemblance to scarf and arf and Nerf doesn’t really play in all that much (well, depending on the scarf you have in mind); the sn onset gives a general sense of having to the do with the nose, which may or may not be relevant, depending on the sense you go with. The sound of sniff has had some effect at times.
Among the set of people I generally hang with, snarf usually means “have liquid that you are drinking come out your nose because you suddenly laugh.” As in “I just about snarfed my beer when I saw that clip of the mayor on the Daily Show.”
However, that is not the definition you will find in Merriam-Webster or The American Heritage Dictionary; it is not one of the several definitions you will find in the Oxford English Dictionary. It is one of the definitions you will find on Urban Dictionary (this one with 781 up votes and 514 down votes). But back to that in a moment.
The normal dictionary definition is ‘eat quickly, voraciously, greedily’ – as in, for instance, “I was a little peckish. I snarfed down a whole box of marshmallow cookies.” And indeed this also gets used. As does another definition you will see in the OED (and, in other wording, on Urban Dictionary, with 392 up votes and 353 down votes – all of these are on Urban Dictionary), “To grab or snatch, esp. without permission; to take or use greedily or rapidly.” As in, “Hey, who snarfed all my cookies?” or “I saw that plate of cookies there, and though I knew it wasn’t for me, I couldn’t help snarfing it down in 20 seconds flat.” Both of these are mid-20th-century definitions, as far as the citations show (this word seems to have appeared in the late 1950s to early 1960s, if we go by the citations). So it has meanings rather like snaffle and the rapid-eating sense of scarf, which is also related to the rapid-eating sense of scoff.
But then there’s the meaning I encountered in my youth when reading Kurt Vonnegut (yes, I was one of those kids who read Vonnegut in high school; I’m sure that tells you a fair bit right there). Vonnegut, in an interview originally published in The Paris Review, recalled his time in high school working on the school newspaper:
…one time, while I was writing, I happened to sniff my armpits absentmindedly. Several people saw me do it, and thought it was funny—and ever after that I was given the name “Snarf.” In the annual for my graduating class, the class of 1940, I’m listed as “Kurt Snarfield Vonnegut, Jr.” Technically, I wasn’t really a snarf. A snarf was a person who went around sniffing girls’ bicycle saddles.
So the sniffing of armpits lent itself readily enough to this word, which was at the time entirely in the unregulated domain of teenage slang. But there was, at least in Vonnegut’s mind, from having been told by someone, an official definition: “a person who [goes] around sniffing girls’ bicycle saddles.”
And yes, that definition, too, is in Urban Dictionary. Or, to be precise, the definition there is “a person who gets off smelling bicycle seats.” It has 70 up votes and 61 down votes.
Notice, by the way, that Vonnegut graduated nearly 20 years before the first citation in the OED, and yet the word was current slang among his set then. The printed evidence does take time to catch up. Especially with teenagers.
And it seems reasonable enough to think that this word originated with teenagers. They like coming up with new words to solidify their sense of being part of an in-group, and they also have a liking for terms that name some particular thing (real or imagined) they find amusing but unnamed.
So, naturally, they are heavy readers of and contributors to Urban Dictionary. You will often find references that make sense only to a barely pubescent set, and usually some made-up definition that just seems like a fun thing but that has never actually been used in real life. And the votes up and down have as much to do with what’s cool or funny as with what’s accurate.
So we see that the first and third definitions (of 7) on Urban Dictionary for snarf relate to a character of that name on the TV show ThunderCats. These definitions have between them more up votes and a better up-to-down ratio than any other definition. And stuffed in with definition 1 is an unrelated second definition as an adjective meaning “sexy and/or stylish.” Odds of seeing or hearing that in real life: not high unless you hang with just the right set of pimply-faced dweebs. And perhaps not even then.
But among adults who have had time to calm down with and about this word? Just the food-related senses, it seems. The dictionaries speculate that snarf is related to scarf (as in ‘eat quickly’), which comes from scoff (same sense), which comes from or is related to scaff (same sense), which comes from Mars for all anyone knows. But clearly the word had to have emerged from somewhere into the adolescent world of the 1930s, to land on Kurt Vonnegut, and the sense it had at the time was not the sense it has now. There may have been some change between now and then due to sound similarities; the normal course of semantic shift can lead from fetishism to famishment to farcical snafu.
And we should remember that Vonnegut, having been an adolescent at the time, would not necessarily be the most reliable informant for standard usages of the then and there… Read his definition of twerp, immediately following the one for snarf in the same article, and judge for yourself. But maybe snarf down your food and bev first, lest you snarf while reading it.