if (conj.) Look up if at Dictionary.com
Old English gif (initial g- in Old English pronounced with a sound close to Modern English -y-), from Proto-Germanic *ja-ba (cognates: Old Saxon, Old Norse ef, Old Frisian gef, Old High German ibu, German ob, Dutch of "if, whether"), from PIE pronomial stem *i- [Watkins]; Klein, OED suggest 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.