吃定心丸

chīdìngxīnwán ㄔ ㄉㄧㄥˋ ㄒㄧㄣ ㄨㄢˊ

    • to be reassured by a promise or guarantee
    • to have one's mind put at rest

Character by character

Saying俗语俗語

Idiom惯用语 · In everyday use

Literally
take a calming pill
Meaning
To be reassured by a promise or guarantee. Like "put one's mind at rest."

Sources