Definition of plain adverb from the Oxford Advanced Learner's Dictionary

 

plain

 adverb
adverb
BrE BrE//pleɪn//
 
; NAmE NAmE//pleɪn//
 
 
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(informal) used to emphasize how bad, stupid, etc. something is plain stupid/wrong Word Origin Middle English: from Old French plain, from Latin planus, from a base meaning ‘flat’.
See the Oxford Advanced American Dictionary entry: plain