if

Etymology

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”).

conj

1. Supposing that, assuming that, in the circumstances that; used to introduce a condition or choice.
   If it rains, I shall get wet.
   I'll do it next year —if at all.
2. (computing) In the event that a statement is true (a programming statement that acts in a similar manner).
   If A, then B, else C.
3. Supposing that; used with past or past perfect subjunctive indicating that the condition is closed.
   I would prefer it if you took your shoes off.
   I would be unhappy if you had not talked with me yesterday.
   If I were you, I wouldn't go there alone.
4. Supposing that; given that; supposing it is the case that.
   If that's true, we had better get moving!
5. Although; used to introduce a concession.
   He was a great friend, if a little stingy at the bar.
   She won her team's admiration, if not its award, for her performance.
6. (sometimes proscribed) Whether; used to introduce a noun clause, an indirect question, that functions as the direct object of certain verbs.
   I don't know if I want to go or not.
   Quoth Matthew, “ […] / She doubts if two and two make four, / […] ” 1715–1717, Matthew Prior, Alma; or, The Progress of the Mind, Canto III
   The former was divided between admiration of the brilliancy which exercices had given to her complexion, and doubt as to (if) occasion's justifying her coming so far alone. 1813, Jane Austen, chapter 7, in Pride and Prejudice
   It is doubtful if the Victorian Londoner needed any warning, for the artful mobsmen, toolers, whizzers and dippers, together with their stickman accomplices, were everywhere in the crowds, in the underground, on railway trains […] 1976, Michael Harrison, Beyond Baker Street: A Sherlockian Anthology, page 117
7. (usually hyperbolic) Even if; even in the circumstances that.
   1837-39, Charles Dickens, Oliver Twist “Wait a minute!” said the girl: “I wouldn’t hurry by, if it was you that was coming out to be hung, the next time eight o’clock struck, Bill. I’d walk round and round the place till I dropped, if the snow was on the ground, and I hadn’t a shawl to cover me.”
   If it’s the last thing I do / If it takes me from Tubilo to Timbuktu / If it’s the last thing I do / I’m gonna dodge every road block, speed trap, county cop / To get my hands on you / If it’s the last thing I do. 2004, David Lee Murphy and Kim Tribble (writers), Montgomery Gentry (singers), “If It’s The Last Thing I Do” (song), in You Do Your Thing (album)
8. Introducing a relevance conditional.
   I have leftover cake if you want some.

noun

1. (informal) An uncertainty, possibility, condition, doubt etc.
   1709, Susannah Centlivre, The Busy Body, Act III, in John Bell (ed.), British Theater, J. Bell (1791), page 59, Sir Fran. Nay, but Chargy, if——— ¶ Miran. Nay, Gardy, no Ifs.——Have I refus'd three northern lords, two British peers, and half a score knights, to have put in your Ifs?
   Well might Bergman add, (in his Sciographia,), “if the compariſon that has been made, &c. be juſt.” The preſent writer makes no ifs about the matter, and has ſuperadded a little inaccuracy of his own, […] 1791 January, “Richardſon’s Chemical Principles of the Metallic Arts”, in The Monthly Review, R. Griffiths, page 176
   Even if they managed to strike Japan, the United States or South Korea with nuclear weapons — a big if, given that they do not have a reliable delivery system — they could not save themselves from ultimate defeat. April 9, 2013, Andrei Lankov, “Stay Cool. Call North Korea’s Bluff.”, in New York Times