noun
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.
彼はまさに獅子身中の虫だった。
He was truly a snake in our bosom.
組織を守るには、獅子身中の者を見抜かねばならない。
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.