"in case that; granting, allowing, or supposing that; on condition that;" also "although, notwithstanding that," Old English gif (initial g- in Old English pronounced with a sound close to Modern English -y-) "if, whether, so," from Proto-Germanic *ja-ba (source also of Old Saxon, Old Norse ef, Old Frisian gef, Old High German ibu, German ob, Dutch of "if, whether"), of uncertain origin or relation. Perhaps from PIE pronominal stem *i- [Watkins]; but Klein, OED suggest it probably originally from an oblique case of a noun meaning "doubt" (compare Old High German iba "condition, stipulation, doubt," Old Norse if "doubt, hesitation," Swedish jäf "exception, challenge"). As a noun from 1510s.
"farewell, good-bye," 1875, from Japanese, said to mean literally "if it is to be that way," from sayo "that way," + nara "if."
"as if, as it were," used in introducing a proposed or possible explanation, late 15c., a Latin word used in Latin in hypothetical comparisons, "as if, just as if, as though;" in real comparisons "just as, as;" and in approximation, "somewhat like, nearly, not far from." It is 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).
1818, phonological spelling of Arabic in sha Allah "if Allah wills (it)."
"rather stiff," 1858, a dictionary word, as if from a diminutive of Latin rigidus (see rigid).
1785, an absurd perversion of (Welsh) rabbit, as if from rare (adj.) + bit (n.). See Welsh.