na-adjective
Describes someone who is very popular with the opposite sex, often in a romantic or attractive sense. Colloquial and slightly playful.
See also: モテる
彼は大学でモテモテだ。
He's really popular at university.
あの新入社員、女
I hear that new employee is really popular with the female staff.
モテる is the verb meaning 'to be popular (with the opposite sex)'; モテモテ is an adjectival noun emphasizing a high degree of popularity.
Reduplication of the verb モテる (to be popular), with the repeated element adding emphasis. The exact derivation is uncertain, but it likely emerged as colloquial slang.