But
Old English be-ūtan, būtan, būta ‘outside, without, except’ (see by, out).
wiktionary
From Middle English but, buten, boute, bouten, from Old English būtan(“without, outside of, except, only”), equivalent to be- + out. Cognate with Scots but, bot(“outside, without, but”), Saterland Frisian buute(“without”), West Frisian bûten(“outside of, apart from, other than, except, but”), Dutch buiten(“outside”), Dutch Low Saxon buten(“outside”), German Low German buuten, buute(“outside”), obsolete German baußen(“outside”), Luxembourgish baussen. Compare bin, about.
Eclipsed non-native Middle English mes(“but”) borrowed from Old French mes, mais (> French mais(“but”)).
etymonline
but (adv., prep.)
Old English butan, buton "unless; with the exception of; without, outside," from West Germanic *be-utan, a compound of *be- "by" (see by) + *utana "out, outside; from without," from ut "out" (see out (adv.)). Not used as a conjunction until late Old English, "on the contrary." Senses attested in early Middle English include "however, yet; no more than." As an introductory expression, early 13c. As a noun, "an objection, an exception" from late 14c.
Som man preiseth his neighebore by a wikked entente, foralwey he maketh a 'but' at the laste ende, that is digne of moore blame than worth is al the preisynge. [Chaucer, "Parson's Tale"]
