Sic is not an English word, it’s a Latin word. So, it’s normally italicized when it appears in English text. It’s Latin for "thus". People put it in brackets when quoting text to indicate that what might appear to be a transcription error is actually faithful to the original. It’s short for sic erat scriptum, which is Latin for "Thus it was written" or "It was written this way."
Sic is an adverb in Latin, but it’s not normally put into an English sentence. It’s not like pace, mutatis mutandis, ceteris paribus, i.e., e.g., etc., which commonly do appear untranslated within English sentences and perform English grammatical roles. Sic usually goes in brackets all by itself, right after the word or passage that a reader might reasonably think is a transcription error, like this:
The monster’s diary said on p. 52, "Dr. Frankenstein’s work effected [sic] my judgement."
A reader might think this is a common mistake: confusing affect for effect. But the monster really does mean effect as a verb, since Dr. Frankenstein’s work really did accomplish the creation of the monster’s ability to judge.
Bryan Garner has some comments about its use in legal writing here, including some common ways that people use it disparagingly or ironically, such as to unnecessarily point out a spelling error in a quotation. That’s really an abuse of sic. It's proper to correct typographical errors when quoting, unless an error is material to the reason you’re quoting it. Garner suggests that the insulting usage of [sic] is currently the most common; the need for the ordinary, legitimate use of [sic] is pretty unusual.
Of course, this is English, so if you want to use sic in some new, creative way, like as an adverb in an English sentence, or even as a verb with a terminal -ing, you can. It’s best to become familiar with the customary usage first, of course, so you can judge how far you can stretch existing custom and still be understood (and not be thought to be making an error, which someone will no doubt insultingly quote with a [sic]—sic’ing it to you, you might say).