idem

id- + -em ? | i- + -dem ? | "ist-sich", "dasselbe, derselbe, dieselbe"; alat. id, ea, is  "ist" + alat. im, em, dt. "-isch"

From Proto-Italic *izdim; equivalent to is (“he”) + Proto-Italic *-im (emphatic marker) (whence Sabellic *-om, Oscan 𐌝𐌔𐌝𐌃𐌖𐌌 (ísídum), 𐌄𐌔𐌝𐌃𐌖𐌌 (esídum)), from Proto-Indo-European *im (whence also Old Latin im, em), accusative singular of *éy (so both parts are from the same source). The s was lost and the i lengthened by compensatory lengthening.

When is' ablative cases eōd, eād became eō, eā, idems ablative 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).

Pronunciation

• (Classical) IPA^(key): /ˈiː.dem/, [ˈiːd̪ɛ̃ˑ]
• (modern Italianate Ecclesiastical) IPA^(key): /ˈi.dem/, [ˈiːd̪em]

Pronoun

īdem (feminine eadem, neuter idem); demonstrative pronoun (with m optionally → n in compounds) with an indeclinable portion

1. the same [ quotations ▼]

Declension

Irregular declension. Similar to the declension of is, ea, id. Demonstrative pronoun (with m optionally → n in compounds) with an indeclinable portion.

Number	Singular				Plural
Case / Gender	Masculine	Feminine	Neuter		Masculine	Feminine	Neuter
Nominative	īdem	eadem	idem		īdem1	eaedem	eadem
Genitive	eiusdem ejusdem				eōrundem eōrumdem eundem eumdem	eārundem eārumdem	eōrundem eōrumdem eundem eumdem
Dative	eīdem2 e͡idem ēīdem	eīdem2 e͡idem ēīdem eaedem	eīdem2 e͡idem ēīdem		eīsdem1 iīsdem īsdem	eīsdem1 iīsdem īsdem eābusdem	eīsdem1 iīsdem īsdem
Accusative	eundem eumdem	eandem eamdem	idem		eōsdem	eāsdem	eadem
Ablative	eōdem	eādem	eōdem		eīsdem1 iīsdem īsdem

¹The nom./dat./abl. plural forms regularly developed into a monosyllable /iː(s)/, with later remodelling - compare the etymology of deus. This /iː/ was normally spelled as EI during and as II after the Republic; a disyllabic iī, spelled II, Iꟾ, appears in Silver Age poetry, while disyllabic eīs is only post-Classical. Other spellings include EEI(S), EIEI(S), IEI(S).
²The dat. singular is found spelled EIEI (here represented as ēī) and scanned as two longs in Plautus, but also as a monosyllable. The latter is its normal scansion in Classical. Other spellings include EEI, IEI.