late 15c., Latin, in hypothetical comparisons, "as if, just as if, as though;" in real comparisons "just as, as;" in approximation, "somewhat like, nearly, not far from;" from quam "as" relative pronominal adverb of manner (from PIE root *kwo-, stem of relative and interrogative pronouns) + si "if," from PIE pronominal stem *swo- "so" (see so).