Yours, Etc.: Origins and Uses of 8 Sign-Offs
Old English deore "precious, valuable, costly, loved, beloved," from Proto-Germanic *deurjaz (cf. Old Saxon diuri, Old Norse dyrr, Old Frisian diore, Middle Dutch dure, Dutch duur, Old High German tiuri, German teuer), ultimate origin unknown. Used interjectorily since 1690s. As a polite introductory word to letters, it is attested from mid-15c. As a noun, from late 14c., perhaps short for dear one, etc.