From Middle Korean엇〮 (Yale: -és-, stative continuous suffix), which is a simplification of 엣〮 (Yale: -éys-, stative continuous suffix). Both forms appeared in the 15th century as contractions of 어〮잇 (Yale: -é ìs-) with auxiliary 잇다〮 (Yale: ìs-tá). The past tense meaning developed from a seventeenth-century semantic shift of "one exists in a state of having VERBed" to "VERBed". Hence doublet of 어 있다(-eo itda).
For vowel harmony, contractions, and allomorphy, see 아/어/여(-a/eo/yeo).
Usually 었(-eot-) precedes all verbal endings except the honorific suffix. However, sometimes in the colloquial speech 었(-eot-) can be preceded by 겠(-get-), although this is considered nonstandard and unnatural in most contexts.
While 었(-eot-) carries a perfect aspect meaning, 었었(-eosseot-) carries a meaning of completion of an action and its difference from the past. Thus, while 죽었다(jugeotda) means he died and is still dead, 죽었었다(jugeosseotda) means he died and is no longer in that state. These two suffixes can also be easily compared with the verb 가다(gada), simple past tense of which expresses that one went somewhere and is still possibly there, while the double past tense usually expresses that one went somewhere, stayed there and is no longer there or came back.