treach ‧ e ‧ ry /ˈtretʃəri/ BrE AmE noun ( plural treacheries )
[ Date: 1100-1200 ; Language: Old French ; Origin: trecherie , from trechier , trichier ; ⇨ ↑ trick 1 ]
1 . [uncountable] behaviour in which someone is not loyal to a person who trusts them, especially when this behaviour helps that person’s enemies:
the treachery of those who plotted against the king
2 . [countable usually plural] a disloyal action against someone who trusts you