できる

/ideku/ → /deku/ → /dekuru/ → /dekiru/

Originally appears in the Man'yōshū of 759 CE and other Old Japanese texts as 出で来 (ideku), as a compound of 出で (ide, “going out”, the 連用形 (ren'yōkei, “continuative or stem form”) of verb 出づ idu, izu, the ancient form of modern 出る deru, “to come or go out of something”) +‎ 来 (ku, “to come”, the ancient form of modern 来る kuru, “to come”). The original meaning was “to come out”, extending in figurative contexts to a sense of “to appear, to become manifest”.

Phonologically, ideku shifted to deku by the mid-1400s in the Muromachi period, as the low-tone initial i dropped out, vaguely similar to the way that unstressed syllables in English may disappear. This deku then underwent a common shift in Muromachi Japanese to become dekuru, where all bigrade verbs and irregular verbs underwent a shift that included a -ru on the end of the plain form, in part through a fusion of the 連体形 (rentaikei, “attributive form”) that has always ended in -ru for these verbs, and the 終止形 (shūshikei, “terminal form”) that is used at the end of sentences.

As a later part of this same shift, the central vowel shifted from -u- to -i- to produce dekiru, as the main vowel -i- for the 連用形 (ren'yōkei, “continuative or stem form”) came to be used for all conjugated forms.

Over this same period, the meaning gradually shifted from “to come out” to include “to appear, to become manifest”, and from there “to come into existence anew”. This then extended further to “to occur, to happen; to be born; to be created, to be produced”, and thence “to come to fruition; to be completed, to be finished (with positive overtones)”. The underlying sense of latent existence gave rise to the meaning of “to be capable of making or producing something”, and then simply “to be able to do”.

In slang contexts in the early 1800s, dekiru came to mean “to have sex”, perhaps similar to the development of the English phrases “to do it, to get some, to get it on”.

• (Tokyo) でき​る [dèkíꜜrù] (Nakadaka – [2])[1][2][3]
• IPA^(key): [de̞kʲiɾɯ̟ᵝ]

できる • (dekiru) intransitive ichidan (stem でき (deki), past できた (dekita))

1. to be able to do

   ジョンさんはフランス語(ご)ができる。

   Jon-san wa Furansugo ga dekiru.

   John knows French.

   (literally, “John can do French.”)

2. to be finished, to be ready
   • 1949, 豊島与志雄, 或る作家の厄日:

     準(じゅん)備(び)は出(で)来(き)た。

     Junbi wa dekita.

     Preparations are finished. → I'm ready.

3. be made
   • 1931, 宮本百合子, なぜソヴェト同盟に失業がないか？:

     都(と)会(かい)に工(こう)場(じょう)はドシドシ出(で)来(き)る。

     Tokai ni kōjō wa doshidoshi dekiru.

     Factories are being built all over the city.

4. (slang) to have sex, to do it, to get some, to get it on

   Synonyms: see Thesaurus:性交する

   彼(かの)女(じょ)と出(で)来(き)たことは一(いち)度(ど)もない。

   Kanojo to dekita koto wa ichido mo nai.

   I haven't done it with my girlfriend even once.

• Shōgaku Tosho (1988) 国語大辞典（新装版） [Unabridged Dictionary of Japanese (Revised Edition)] (in Japanese), Tōkyō: Shogakukan, →ISBN
• Matsumura, Akira, editor (2006), 大辞林 [Daijirin] (in Japanese), Third edition, Tōkyō: Sanseidō, →ISBN