Late 18th century (in an earlier sense). Origin uncertain: perhaps alteration of lump, or perhaps a specific use of the singular form corresponding to mumps.

Late 16th century (in an earlier sense). Apparently from an imitative base of Germanic origin. Compare Dutch mompen to mumble (rare, poetic), German †mumpfen to chew with a full mouth, Icelandic mumpa to cackle, to mutter, murmur, mumpra to eat greedily, to mumble, to distort the mouth, Norwegian regional mompe, mumpe to chew with a full mouth, stuff oneself.

Mid 17th century (in an earlier sense). Apparently from Dutch mompen to cheat, deceive, conceal, of uncertain origin; perhaps related to mompen to mumble, though the nature of the relationship is unclear.