Ridiculous; Random; Silly usually to describe another person or image.

Awkward or embarrassing, especially pertaining to a person.

adjective

accident prone; perhaps so much so that it makes one look foolish

Clumsy and foolish

Clumsy or uncoordinated; prone to cause accidents likely due to some, often humourous, mental lapse.

Not "all there in the head"; possibly lacking intelligence or common sense. The wall-eyed background character in "My Little Pony: Friendship is Magic", Derpy Hooves, is named after this.

A disease (perhaps contagious) symptomized by using continuous "derp" actions.