Definition of fallacious adjective from the Oxford Advanced Learner's Dictionary

 

fallacious

 adjective
adjective
BrE BrE//fəˈleɪʃəs//
 
; NAmE NAmE//fəˈleɪʃəs//
 
(formal)
 
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wrong; based on a false idea a fallacious argument Word Origin early 16th cent.: from Old French fallacieux, from Latin fallaciosus, from fallacia, from fallax, fallac- ‘deceiving’, from fallere ‘deceive’.
See the Oxford Advanced American Dictionary entry: fallacious