expression, noun
snake in one's bosom; treacherous friend
Idiomatic expression for a person who betrays the group they belong to, like a parasite or enemy within. Often used in political, organizational, or literary contexts.
彼はまさに獅子身中の虫で、内部から組織を壊そうとしていた。
He was truly a snake in the bosom, trying to destroy the organization from within.
The phrase 'a worm in the lion's body' refers to a traitor within one's own circle.