From en.wiktionary.org:
** English
*** Conjunction
[en]
1. [en] Unless . [13th]
2. * {{ quote-text | en | year=1485 | author=w:Sir Thomas Malory | title=Le Morte Darthur | section=Book VIII
|passage=And therewithall he swange oute a swerde and seyde, ‘ BUT YF thou
telle me all who hath ben here, now shalt thou dey!’}}
1. * [passage=Ne living aide for her on earth appeares, / BUT-IF the heavens helpe to redresse her wrong, / Moved with pity of her plenteous teares.]