Other forms: sicced; sics; siccing
When urging your dog to chase a thief, you might say, "Sic 'em, Fluffy!" When writing something that might look wrong, you add "[sic]" to indicate you know it's wrong.
When you’re writing a paper and you want to quote a source, but the source contains a misspelling or some other error, you use the word [sic] after the quote so that whoever reads your paper knows the mistake is in the original source, rather than in your own sloppy typing. It’s kind of a way of saying, “Yeah, I know this is wrong, but let’s just be clear — I didn't do it!”