conjunction

1. used to introduce a phrase or clause contrasting with what has already been mentioned. "he stumbled but didn't fall" синонимы: yet, nevertheless, nonetheless, even so, however, still, notwithstanding, despite that, in spite of that, for all that, all the same, just the same, at the same time, be that as it may, though, although, still and all, withal, natheless, howbeit
2. used to indicate the impossibility of anything other than what is being stated. "one cannot but sympathize" синонимы: (do) other than, otherwise than, except
3. used to introduce a response expressing a feeling such as surprise or anger. "but that's an incredible saving!"
4. used after an expression of apology for what one is about to say. "I'm sorry, but I can't pay you"
5. without it being the case that. "it never rains but it pours"

preposition

except; apart from; other than. "we were never anything but poor" синонимы: except (for), apart from, other than, besides, aside from, with the exception of, short of, bar, barring, excepting, excluding, omitting, leaving out, save (for), saving, outside of

adverb

1. no more than; only. "he is but a shadow of his former self" синонимы: only, just, simply, merely, no more than, nothing but, a mere, nobbut
2. (used at the end of a sentence) though; however. "he was a nice bloke but"

noun

an argument against something; an objection. " no buts —just get out of here"