a tradimento

Italian edit

Literally, “at betrayal”.

in an unexpected way; by surprise
Synonyms: di sorpresa, alla sprovvista, in castagna, alle spalle

una domanda a tradimento ― an unexpected question
by treachery; treacherously
Synonym: alle spalle

Fu colpito a tradimento.
He was stabbed in the back.
(literally, “He was hit treacherously.”)