Entries linking to si
insertion in printed quotation to call attention to error in the original; Latin, literally "so, thus, in this way," related to or emphatic of si "if," from PIE root *so- "this, that" (source also of Old English sio "she"). Used regularly in English articles from 1876, perhaps by influence of similar use in French (1872).
[I]t amounts to Yes, he did say that, or Yes, I do mean that, in spite of your natural doubts. It should be used only when doubt is natural; but reviewers & controversialists are tempted to pretend that it is, because (sic) provides them with a neat & compendious form of sneer. [Fowler]
Sic passim is "generally so throughout."