First record is from a 12th cent. Arabic document from Toledo in the name Dumínqu Al-Múĝu, which was explained in the 13th cent. as an attempt to render the name Domingo Mocho into Arabic. Otherwise, the earliest attestation in Spanish is from the 13th cent. Perhaps from Vulgar Latin *mutius 'id.,' from Latin mutilus "mutilated," but also "truncated."
Of unknown origin.
Also the origin of the surnames Mocho and Mochos.
Basque mutz, motz "short," "ugly," "bare," borrowed from Vulgar Latin *mutius