Sic is from Latin. In that language, it meant "so" or "thus."
In written English, sic is inserted in quoted passages to signify that a word is being quoted exactly as found, even if it is spelled incorrectly or a grammatical error or an archaic usage.
The convention style in using sic is to put it in square brackets, like so: [sic]. Less commonly, the word is put into standard parentheses, like so: (sic).
It's also a conventional style in using sic to italicize it.
An example of sic, as cited in Wordnik.com: "Thence they sailed along the shore until we [sic] reached 40 deg." —Henry Hudson