If
Old English gif, of Germanic origin; related to Dutch of and German ob .
wiktionary
From Middle English if, yif, yef, from Old English ġif(“if”), from Proto-West Germanic *jabu, *jabē, from Proto-Germanic *jabai(“when, if”). Cognate with Scots gif(“if, whether”), Saterland Frisian af, of(“if, whether”), West Frisian oft(“whether”), Dutch of(“or, whether, but”), Middle Low German ef, if, af, of("if; whether"; > German Low German of), German ob(“if, whether”), Icelandic ef(“if”).
etymonline
if (conj.)
"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.
