but
In addition to the idioms beginning with but, also see all but; all over but the shouting; anything but; can't help but; close but no cigar; everything but the kitchen sink; it never rains but it pours; last but not least; no ifs or buts; nothing but; sadder but wiser; separate but equal; slow but sure; spirit is willing but the flesh is weak; there but for the grace of god.