Source : Webster's Revised Unabridged Dictionary (1913)

If \If\, conj. [OE. if, gif, AS. gif; akin to OFries. ief, gef,
   ef, OS. ef, of, D. of, or, whether, if, G. ob whether, if,
   OHG. oba, ibu, Icel. ef, Goth. iba, ibai, an interrogative
   particle; properly a case form of a noun meaning, doubt (cf.
   OHG. iba doubt, condition, Icel. if, ef, ifi, efi), and
   therefore orig. meaning, on condition that.]
   1. In case that; granting, allowing, or supposing that; --
      introducing a condition or supposition.

            Tisiphone, that oft hast heard my prayer, Assist, if
            [OE]dipus deserve thy care.           --Pope.

            If thou be the Son of God, command that these stones
            be made bread.                        --Matt. iv. 3.

   2. Whether; -- in dependent questions.

            Uncertain if by augury or chance.     --Dryden.

            She doubts if two and two make four.  --Prior.

   {As if}, {But if}. See under {As}, {But}.