IPA: //ˈtriːzənəbli//
KK: /ˈtrizənəbli/
In a way that betrays one's country or government, often by acting against its interests.
He acted treasonably by sharing classified information with foreign agents.
Treasonably → It is formed from "treason" (from Old French *traïson*, meaning betrayal or treachery) and "-ably" (meaning in a manner that is). The word "treasonably" means in a manner that is characterized by betrayal or treachery.
Think of 'treason' as betrayal and remember that 'treasonably' describes actions done in a manner that betrays trust.