From Wiktionary, the free dictionary
in den Tee tun
- Used other than figuratively or idiomatically: see in, den, Tee, tun.
Hast du mir schon Zucker in den Tee getan?- Have you already put sugar in my tea?
- (idiomatic) to slip something in someone’s drink (e.g. alcohol, drugs, poison)
Er ist zu sehr passender Zeit gestorben. Wer weiß, was seine Frau ihm in den Tee getan hat!- He died at a very convenient time. Who knows what his wife may have put in his drink!
- Hence used in rhetorical questions referring to someone’s unusual behaviour.
Was haben sie dir denn in den Tee getan, dass du heute so fleißig bist?- What did they put in your tea to make you work so hard today?
