ichidan verb, intransitive verb
act lovestruck; fawn; act affectionate
Manga/anime slang for a character who is normally cold or tough suddenly showing a soft, affectionate, or lovestruck side, often toward a love interest. Related to the onomatopoeia でれでれ.
See also: でれでれ
ツンデレの彼女が急にデレた。
My tsundere girlfriend suddenly acted all lovey-dovey.
あのクールな先輩がデレるところを初めて見た。
I saw that cool senior acting affectionate for the first time.