Originally the dative singular form, him and its reflexes gradually spread to uses that had historically shown the accusative (compare notes at senses 1b and 2), replacing hi pron.2 in these uses (compare hi pron.2 II.), before being in turn replaced as stressed form by them pron. in most varieties by the end of the Middle English period. In early modern English (and later in informal or colloquial use and in regional varieties) 'em largely functions as an unstressed form corresponding to them, although it is of distinct origin. (On the form 'em as a variant of them pron. in some modern Scots varieties see ζ. forms at them pron., adj., and n. and discussion at that entry.)
Form history.
In Old English originally formally identical with the dative singular masculine and neuter him him pron. (see Forms 1α). (The chiefly late West Saxon form hym probably reflects laxing of i in him in low stress.)
In later Old English, forms with a diphthong are attested, most frequently as heom (see Forms 1β); these apparently occur earliest in Kentish and are also attested in later Mercian, but become frequent from the 11th cent. onwards, being well attested in late West Saxon. They are probably influenced by forms of the genitive plural showing back mutation such as heora (see discussion at her pron.1 and adj.1) and therefore are likely to have had a short diphthong rather than the long diphthong of Old English feminine nominative singular hēo hoo pron. or nominative and accusative plural hēo (see hi pron.2). The late Old English form heam at Forms 1β shows unrounding of the second element of the diphthong probably due to low stress. Occasional Middle English forms that appear to show long vowels (compare heem at Forms 1γ) are probably due to analogy with other case forms of the pronoun in Middle English; compare the Middle English forms of hoo pron. and her pron.1 The spread of the diphthongal forms once they had arisen is usually attributed to systemic pressure, i.e. the need felt by speakers to differentiate between singular and plural in the dative.
In Middle English it is mainly the diphthongal forms that are continued from Old English. Middle English forms such as him, hym (see Forms 1α) are rare, and some of the few apparent attestations may be due to scribal confusion with the singular (i.e. him pron.). Forms such as hem (see Forms 1γ) chiefly reflect the expected monophthongization and unrounding of heom. The form hem is apparently first attested in Old English in the Taunton Fragment (see quot. OE at Forms 1γ), but is probably to be interpreted differently in that source; compare discussion of the form here at her pron.1 and adj.1 Middle English forms such as heom, hoem, etc. (see Forms 1β, Forms 1η) show early and western preservation of rounding of the monophthongized vowel, and so do some of the forms at Forms 1δ such as late Old English hom, although others are probably to be explained in the same way as Middle English hore, hor (compare discussion at her pron.1 and adj.1) or as resulting from the influence of such forms. With forms such as ham (see Forms 1ε) and ȝem (see Forms 1θ) compare discussion of parallel forms of her pron.1 and adj.1 at that entry. The early form ham in quot. c1175 at Forms 1ε, however, is perhaps influenced by following bām, dative plural of bēgen (see bo adj.)
The early attestation and eventual prevalence of loss of initial h- (see Forms 1ζ) is due to low stress; similar forms are found for other pronouns of the third person, as a pron., it pron., and the forms cited at Forms 1β, 1η at he pron.