Etymology Dictionary

BUT (1),  prep. and conj. except.  (E.)   M. E. bute, Havelock, 85; buten, Layamon, l. 23.—A. S. bútan, conj. except, prep. besides, without; contr. from be-útan, Grein, i. 150.   The full form biutan is frequently found in the Heliand, e.g. in l. 2188; and even biutan that, unless, l. 2775.   β. Be = by; útan = outward, outside; bútan = 'by the outside,' and so 'beyond,' 'except.' + Du. buiten, except.  B. The form útan is adverbial (prob. once a case of a sb.), formed from út, out.   ¶ All the uses of but are from the same source; the distinction attempted by Horne Tooke is quite unfounded.   The form be for by is also seen in the word be-yond, a word of similar formation.   See further under Out.

BUT (2),  to strike; a but-end; a cask.   See Butt (1) and (2).