A difficult or disagreeable person, especially a child. Sometimes used as a scolding term of reproach. Compare "pig".
1940s; earliest use found in Dylan Thomas (1914–1953), poet. From Welsh mochyn pig, greedy or immoral person, singulative form from collective plural moch pigs (also in Old Welsh) from a Celtic base (goes to Early Irish mucc, Breton moc'h, etc.) of disputed origin.