This is the classic term for a bachelor, an unmarried dude. Your tita's favorite question during family reunions is probably, "O, iho, kailan ka mag-aasawa? Baka tumandang **sultero** ka niyan!" It's the male equivalent of a 'dalagang Pilipina' who's taking her sweet time. In modern terms, he's 'single and not-so-ready-to-mingle' according to your relatives.