Etymology
Borrowed from Japanese ヤンデレ (yandere), a portmanteau of 病んでる (yanderu), contraction of 病んでいる (yande iru), progressive tense of 病む (yamu, “to be sick”), and デレデレ (dere-dere, “in a lovey-dovey, infatuated, or lovestruck manner”, adverb).
Developed on the model of tsundere (“being cold and even hostile towards another person before gradually showing a warm and caring side over time”).