sam1 zi1 tou5 ming4心知肚明

How it's used

This idiom carries a sense of tacit agreement or shared secret knowledge where verbal confirmation is unnecessary. It is frequently used in professional or social settings to acknowledge an unspoken reality, such as a boss's stinginess or a colleague's hidden agenda. The phrase is often paired with the first half 雞食放光蟲, which literally refers to a chicken eating a firefly, implying that the chicken is glowing from the inside and thus knows exactly what it has swallowed.