出典:Wiktionary
Misinterpretation of -em. When the old ablative cases of is, eōd, eād, became eō, eā, the true forms eōd-em, eād-em were interpreted as eō-dem, eā-dem. The neuter nominative singular id-em is natural and gives earlier emem (= later eundem). The new marker -dem then served to create totidem, tantumdem, ibīdem, etc. Compare tam-en with its later doublet: tan-dem (← *tam-dem).
-dem (not comparable)
that
a
むら
a village
since
どのもの
which
おおわれる
enveloped
がってんだ
Okey-dokes
a seine
to speak with a provincial accent
his
こと
a thing
mambas