安钉子

Traditional (Taiwan): 安釘子

āndīngzi ㄢ ㄉㄧㄥ ˙ㄗ

    • to plant a mole in the other side's ranks

Character by character

Saying俗语俗語

Idiom惯用语 · Widely known

Literally
to plant a nail
Meaning
To secretly place one's own man inside the other side's ranks; to plant a mole.

Sources