This is the 5038th most frequent French word.
traitre
A traitor.
Here, 'traître' is used to describe someone betraying a loyalty or allegiance, specifically in a context involving fidelity to a cause.
Il était un traître à la cause, divulguant des secrets à l'ennemi.
He was a traitor to the cause, divulging secrets to the enemy.
In this sentence, 'traître' is used adjectivally (via derivation) to characterize an action or feature displaying betrayal or deceit.
Son sourire traître masquait ses véritables intentions.
His deceitful smile masked his true intentions.
Here, 'traître' again describes someone who commits a betrayal, emphasizing the action that defines the term.