treachery
n.the act of hurting someone who trusts you, often by helping their enemies. It is a very serious form of lying or betrayal.
n.the betrayal of trust or a deceptive action that violates an established allegiance. Often carries a literary or historical tone, implying a grave moral failing.
“He was shocked by his friend's treachery.”
“The general was executed for treachery after he secretly sold military secrets to the opposing army.”
“The novel explores the psychological toll of treachery, showing how a single act of betrayal can dismantle a community's social fabric and lead to decades of resentment.”
Uncountable when referring to the quality of being treacherous; countable when referring to specific acts of betrayal.
From Middle English trecherie, from Old French tricherie, trecherie, from tricher, trichier (“to cheat”).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.