Definition of but noun from the Oxford Advanced Learner's Dictionary

 

but

 noun
noun
BrE BrE//bʌt//
 
; NAmE NAmE//bʌt//
 
 
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[usually plural] a reason that somebody gives for not doing something or not agreeing ‘Let us have no buts,’ he said firmly. ‘You are coming.’ With so many ifs and buts, it is easier to wait and see. Word Origin Old English be-ūtan, būtan, būta ‘outside, without, except’ (see by, out).
See the Oxford Advanced American Dictionary entry: but