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 [shìásáꜜttè] (Nakadaka – [3])[3]
• IPA^(key): [ɕia̠sa̠t̚te̞]

明明後日(しあさって) • (shiasatte) 

1. [from 1275] (Kansai, western Japan) three days from today, two days after tomorrow

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

2. (Tōkyō, Kantō, Hokkaidō) 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".

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 [myòómyóógóꜜnìchì] (Nakadaka – [5])[3]
• IPA^(key): [mʲo̞ːmʲo̞ːɡo̞ɲ̟it͡ɕi]

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. [from 1603] two days after tomorrow, three days hence