The OED's earliest use isn't necessarily the earliest there is, just the earliest the definition-writers were able to find - they are always on the lookout for earlier ones. One difficulty that occurs to me is that there may have been some overlap with "sot" (for which there are far earlier attestations), which could mean a foolish fellow as well as a drunken one. But I think there's a definite implication that they can't spell it out, because it's rude, which does argue for something more like "shit" or "sod" than like "spy." However well "spy" fits otherwise (and in fact I do think it's one of the most likely possibilities), I wouldn't have thought it would be so unmentionable an insult. And I like the idea of Broster working in a glancing reference to sodomy without saying so.