means The act of betraying someone's trust, especially through deceit by a person you believed was on your side.

from From the Old French 'trecherie,' meaning deceit or cheating, built on the verb 'trichier' — to trick or cheat — which also gives us the modern word 'trick.' So treachery and a card trick share a sly ancestor: both are about fooling someone who didn't see it coming. The deeper roots are murky, possibly tracing back to a Latin or Vulgar Latin word for deceiving, but the trail fades the further back you go.