socius criminis
/ˌsəʊʃ(ɪ)əs ˈkrɪmɪnɪs/Law
noun
An associate in crime; an accomplice.
Origin
Early 17th century. From post-classical Latin socius criminis companion or accomplice in crime (3rd cent.) from classical Latin socius + crīminis, genitive of crīmen.