Wikipedia doesn’t answer, as far as I can tell.
I think it means that they aren’t sure of the spelling but aren’t going to bother looking it up.
Spelling is either incorrect, or the writer thinks it may be.
Would that be (sp?), meaning “I have quite possibly spelled the preceding incorrectly, because I’m not sure of the spelling/am too lazy to look it up”?
As in “I ate two bananananas (sp?) at lunch.” Or: “This is in response to adrew’s (sp?) question.”?
Sometimes you can’t check the spelling. If your idea of the spelling is a bit too far off, the dictionaries can’t direct you to the correct spelling. Sometimes it could be a name or something not likely to be found in a dictionary.
Others may differ. Koran…Qu’ran… Qur’an…etc.
Just sayin’. 
It’s also used when you’re quoting someone else, if you want to indicate that the misspelling was the original writer’s mistake, and not yours.
Actually, I use (sic) to mark things that are spelled wrong or have the wrong grammar. (I use square brackets, but those are for coding here.)
I use (sp) when taking notes on a phone conversation and either the caller has hung up without letting me clarify a spelling, or even when I ask them to spell it again, the connection is so bad I have no idea what they said.
I use (?) when I know I’ve spelled the word I used correctly, but I’m not sure that it’s the word THEY said, it’s just a reasonable interpolation (phone line has lots of noise, caller mumbling, chewing, has their teeth out, what have you).
Corrvin
(sp?) means I want you to know that I know I may have spelled something wrong. It means I’m indifferent to spelling, but not to your opinion of me.
Square brackets around [sic] come out as square brackets around sic. They only are treated as code when the surround one of the code entities like u or b or i, etc.
Oh, good, then when I’m here at home with the computer I can find the square bracket keys on, I can make myself [sic] anytime I want!
(sp) and [sic] are properly used to indicate "reproduced as in original, even though it may be contrary to canonical spelling practices. E.g., “…Aleister Crowley’s ‘ceremonial magick’ [sic]…” Or, "I’m representing phonetically a word I’ve heard and know the meaning of, but have never seen written, as in “Those little appetizer-like treats served at parties – called something like “ordures” (sp?) or “canapays” (sp?)…”
There are, of course, illegitimate uses, as in “I’m too lazy to look this word up” or “Look what this idiot thought was the proper way to spell ‘ecstatic’” but they do indeed have legitimate uses as well.
Stop … don’t do it … no that would be mean … oh what the hell …
And the rest of us even [sic]er. 