treacherous insider; snake in one's bosom
A yojijukugo referring to a trusted person who betrays you from within, like a parasite inside a lion. Often used in literary or cautionary contexts.
He was truly a snake in our bosom.
To protect the organization, we must identify the treacherous insiders.
From a Buddhist parable: a lion (獅子) harbors a parasite (虫) within its body (身中), which eventually kills it. The phrase warns against internal threats disguised as friends.