etymology - Where does the phrase "dead simple" originate?

In expressions like dead simple, dead easy, dead on, or dead right, dead is used as an adverb, meaning completely or extremely.

Etymonline indicates some of the history:

Used from 16c. in adj. sense of "utter, absolute, quite" (cf. dead drunk first attested 1590s; dead heat, 1796). As an adv., from late 14c. Dead on is 1889, from marksmanship.