Old English deore (Anglian diore, West Saxon dyre), "precious, valuable; costly, expensive; glorious, noble; loved, beloved, regarded with affection" from Proto-Germanic *deurja- (source also of Old Saxon diuri "precious, dear, expensive," Old Norse dyrr, Old Frisian diore "expensive, costly," Middle Dutch diere "precious, expensive, scarce, important," Dutch duur, Old High German tiuri, German teuer), a word of unknown etymology. Finnish tiuris, tyyris is from Germanic.
The old sense of "precious, valuable" has become obsolete, but that of "characterized by a high price in consideration of scarcity, absolutely or relatively costly" lingers, though it is perhaps archaic. Used interjectorily (oh, dear; dear me, etc.) indicating pity, surprise, or some other emotion since 1690s, but the intended sense is not clear. As an affectionate address (my dear, father dear), mid-13c. As a polite introductory word to letters, it is attested from mid-15c. The military man's dreaded Dear John letter is attested from 1945. As a noun, from late 14c., perhaps short for dear one, etc.