From Middle Korean엇〮ᄂᆞᆫ〮 (Yale: -é-snón), equivalent to the present adnominal form of the stative continuous mood, e.g. 먹엇〮ᄂᆞᆫ〮 (Yale: mèké-snón, “eaten; which is in an eaten state”). Now grammatically equivalent to 었(-eot-, past-tense marker) + 는(-neun, adnominal suffix).
In colloquial Seoul Korean and in Jeolla Korean, this suffix usually appears before certain expressions conveying doubt or speculation, such as 줄몰랐다(jul mollatda, “I didn't know...”), 것같다(geot gatda, “I feel like...”), and 듯(deut, “as if”).
In such expressions, 었는(-eonneun) has a nuance of emphasizing that one currently has a solid justification for such doubt or speculation.
For adjectives, 었는(-eonneun) can have a nuance of emphasizing both that the state occurred in the past and that it has a continuing impact on the present. Compare simple 은(-eun), which can be either past or present and is hence ambiguous (and out of context usually tends to refer to the present), and 었던(-eot-deon), which can have a nuance that the state no longer pertains to the present.
In Gyeongsang Korean, this suffix is essentially in free variation with 은(-eun). There is apparently no semantic difference between the two.
권미영 (2017), “경상방언 관형형 '었는'의 기원과 실현 양상 연구 [Study of the origin and realization of the Gyeongsang dialect adnominal -essnun]”, in Hanminjok eomunhak, volume 76, pages 1—19
김태우 (2019), “'었는'의 발달에 대한 관견 [A remark on the development of -essnun]”, in Gugeohak, volume 92, DOI:10.15811/jkl.2019..92.006, pages 163—201
