1580s, "form of speech peculiar to a people or place;" meaning "phrase or expression peculiar to a language" is from 1620s; from French idiome (16c.) and directly from Late Latin idioma "a peculiarity in language," from Greek idioma "peculiarity, peculiar phraseology" (Fowler writes that "A manifestation of the peculiar" is "the closest possible translation of the Greek word"), from idioumai "to appropriate to oneself," from idios "personal, private," properly "particular to oneself."

This is from PIE *swed-yo-, suffixed form of root *s(w)e-, pronoun of the third person and reflexive (referring back to the subject of a sentence), also used in forms denoting the speaker's social group, "(we our-)selves" (source also of Sanskrit svah, Avestan hva-, Old Persian huva "one's own," khva-data "lord," literally "created from oneself;" Greek hos "he, she, it;" Latin suescere "to accustom, get accustomed," sodalis "companion;" Old Church Slavonic svoji "his, her, its," svojaku "relative, kinsman;" Gothic swes "one's own;" Old Norse sik "oneself;" German Sein; Old Irish fein "self, himself").

[G]rammar & idiom are independent categories; being applicable to the same material, they sometimes agree & sometimes disagree about particular specimens of it; the most can be said is that what is idiomatic is far more often grammatical than ungrammatical, but that is worth saying, because grammar & idiom are sometimes treated as incompatibles .... [Fowler]

emphatic or reflexive form of third person feminine pronoun, Old English hire self; see her (objective case) + self. Originally dative, but since 14c. often treated as genitive, hence her own sweet self, etc. Also compare himself.

Old English him selfum, from dative/accusative personal pronoun him + self, here used as an inflected adjective.

1630s, "fact of not depending on others or another, self-support and self-government;" see independent + -ence. Earlier in same sense was independency (1610s). U.S. Independence Day (July 4, commemorating events of 1776) is recorded under that name by 1791.

An Old English word for it was selfdom, with self + dom "law," but in form this is closer to privilege (n.). The two concepts are not always distinguishable.

late 14c., from Old English hit sylf, from it + self. Since 17c. usually regarded as its self (thus its own self, etc.).

emphatic or reflexive form of I or me, c. 1500, mi-self, alteration of meself (c. 1200), from Old English phrase (ic) me self, where me is "a kind of ethical dative" [OED]. See my + self. The alteration from meself is by analogy of herself, where her- was felt as genitive (though analogous hisself remains bad form).

1540s, one's self, "a person's self" (without distinction of gender), an emphatic form of one, with self. Hyphenated 18c.; written as one word from c. 1827, on model of himself, herself, myself, itself, etc.

"rarely, not often, infrequently," late Old English and early Middle English seldum, an alteration of seldan "infrequently, rarely," from Proto-Germanic *selda- "strange, rare" (source also of Old Norse sjaldan, Old Frisian selden, Dutch zelden, Old High German seltan, German selten), a word of uncertain etymology. Perhaps ultimately from the base of self (q.v.).

The form shifted apparently on analogy of adverbial dative plurals in -um (as in whilom "at one time," from Old English hwilum, from the source of while). The same development also created litlum from little, miclum from mickle. Also compare random, ransom. The forms in -n decreased from 14c. and faded out in 16c.

Old English seldan had comparative seldor, superlative seldost; in early Middle English, as seldan changed form and lost its connection with these, selde was back-formed as a positive. Shakespeare uses seld-shown "rarely exhibited." Some compounds using the old form survived through Middle English, such as selcouth "rarely or little-known, unusual, strange, wonderful," from Old English selcuð, seld-cuð, from seldan + cuð (see couth). 

 German seltsam "strange, odd," Dutch zeldzaam are cognates of seldom, but with the second element conformed to their cognates of -some. Related: Seldomness.

Seldom-times "rarely, hardly ever" is from mid-15c. (earlier was seld-when, Old English seldhwanne "rarely," which lasted until 16c.). Seldom-seen "rarely encountered" is from mid-15c.; older was seld-seen (Middle English seld-sen, from Old English seldsiene), which lasted long enough to appear in Marlowe (seildsene, 1590s).

word forming element indicating "oneself," also "automatic," from Old English use of self (pron.) in compounds, such as selfbana "suicide," selflice "self-love, pride, vanity, egotism," selfwill "free will." Middle English had self-witte "one's own knowledge and intelligence" (early 15c.).

OED counts 13 such compounds in Old English. Middle English Compendium lists four, counting the self-will group as a whole. It re-emerges as a living word-forming element mid-16c., "probably to a great extent by imitation or reminiscence of Greek compounds in (auto-)," and formed a great many words in the pamphlet disputes of the 17c.

also self concept, in psychology, "a person's idea of himself," 1921, from self + concept.