The only free and comprehensive online etymological dictionary of the Spanish language
muchacho (Noun) "boy," "girl"
13th cent. Old Spanish mochacho. Frequently occuring in as a surname in the earliest texts. Probably from mocho and -acho in the sense of comparing the young to calves yet to grow their proverbial "horns." Note that in some texts where we find the surname Mochocho, we also find Mocho.
Also the origin of the surname Mochacho.