Etymology: Middle English, from Old English būtan, preposition & conjunction, outside, without, except, except that; akin to Old High German būzan without, except; akin to Old English be by, ūt out — more at by, out
Date: before 12th century
a. except for the fact <would have protested but that he was afraid> b. that — used after a negative <there is no doubt but he won> c. without the concomitant that <it never rains but it pours> d. if not ; unless e. than <no sooner started but it stopped> — not often in formal use 2. a. on the contrary ; on the other hand ; notwithstanding — used to connect coordinate elements <he was called but he did not answer> <not peace but a sword> b. yet <poor but proud> c. with the exception of — used before a word often taken to be the subject of a clause <none but the brave deserves the fair — John Dryden> II. preposition Date: before 12th century 1. a. with the exception of ; barring <no one there but me> — compare but I,2c b. other than <this letter is nothing but an insult> 2. Scottish a. without, lacking b. outside III. adverb Date: 12th century 1. only, merely <he is but a child> 2. Scottish outside 3. to the contrary <who knows but that she may succeed> 4. — used as an intensive <get there but fast> IV. pronoun Date: 1556 that not ; who not <nobody but has his fault — Shakespeare> V. noun Etymology: Scots but, adjective (outer) Date: 1724 Scottish the kitchen or living quarters of a 2-room cottage
New Collegiate Dictionary. 2001.