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.
内通者 is a more neutral, modern term for an informant or mole, while 獅子身中 carries a strong literary and moral condemnation of betrayal from within.
裏切り者 is a common word for a traitor in general, whereas 獅子身中 specifically emphasizes the insider status and the parasitic nature of the betrayal.
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.