Presumably from the past continuous tense inflectional suffix -더- (-deo-, “deo”). Not to be confused with suffixes of similar sound -든 (deun, “deun”), -든지 (deunji, “irrespective of ...”).
던 • (deon)
- past continuous tense marker, making an adjective clause:
- after verb stems, indicates action or state which was interrupted by the occurrence of another action or state
- after verb stems, functions as a retrospective marker, indicates events in the past, often events which happened on a regular basis
- 비가 오면 나가 걷곤 하던 대나무 숲길 (biga omyeon naga geotgon hadeon daenamu supgil, “the lane in the bamboo grove I used to walk along when it rained”)
- after adjective stems, indicates a state which no longer exists