mal ‧ a ‧ prop ‧ is ‧ m /ˈmæləprɒpɪz ə m $ -prɑː-/ BrE AmE noun [countable] literary
[ Date: 1800-1900 ; Origin: Mrs Malaprop character who uses words wrongly in the play The Rivals (1775) by Richard Sheridan, from French mal à propos 'not appropriate' ]
an amusing mistake that you make when you use a word that sounds similar to the word you intended to say but means something completely different