明明後日

Compound of し (shi, of uncertain derivation) +‎ 明後日 (asatte, “the day after tomorrow”).^[1][2][3]

The initial shi element is uncertain. Speculatively, this might be related to, or a shift from, intensifying prefix ひ (hi) found in some adjectives, such as ひ弱(よわ)い (hiyowai, “delicate, very weak”).

First cited to a text from 1275.^[1]

• (Tokyo) しあさって (Nakadaka – )^[3]
• IPA^(key):

明明後日(しあさって) • (shiasatte) 

1. (Tōkyō, Kansai, western Japan) three days from today, two days after tomorrow

   Synonym: (eastern Japan) 弥の明後日 (yanoasatte)

2. (Kantō, Hokkaidō, Mie, Hida Province) four days from today

This term and 弥の明後日 (yanoasatte) share a roughly complementary distribution between eastern and western Japan, wherein the terms alternate between meaning "three days from today" and "four days from today". Tokyo follows the Western Japan usage.

Compound of 明 (myō, “next, one later”) +‎ 明後日 (myōgonichi, “day after tomorrow”).^[1][2][3]

First cited to the Nippo Jisho of 1603.^[1]

• (Tokyo) みょーみょーごにち (Nakadaka – )^[3]
• IPA^(key):

明(みょう)明(みょう)後(ご)日(にち) • (myōmyōgonichi) ^{←みやうみやうごにち (myaumyaugoniti)?}

1. two days after tomorrow, three days hence