From Spanish llello.
Unknown origin, perhaps from whimsical slang by youth. A prevailing theory, given the term's ultimate origin in Aragon and Catalan-speaking territories, is derivation from Catalan jajo (“grandpa”). It seems the first form generated was *jaja (“grandma”), from which the masculine form was derived. This would come from Catalan avia (“grandmother”) (from Vulgar Latin *aviōla, feminine diminutive of avus (“grandfather”)), and from juvenile palatalization would generate something like *ai̯a. The common phenomenon of syllabic repetition in children's language (cf. papa, baba, etc.) would then cause the form jaja above, which would then be spread into Spanish and masculinized.
Otherwise, perhaps masculinized from Greek γιαγιά (giagiá, “grandmother”).