一昨昨日
さきおととい · sakiototoi · JLPT N2 · noun
yesterday
先 (saki) = before, 一昨 (ototoi) = day before yesterday; stack one more 'before' on top: three days ago
一昨昨日 builds on 一昨日 (day before yesterday) plus one more 昨 (yesterday) in front, so it means the day before that. Read it as 'before the day-before-yesterday'.
一 — single horizontal stroke
一 is one of the simplest pictographs in Chinese, depicting a single horizontal line to represent the number one. It has retained essentially the same form since oracle bone script over 3,000 years ago. By extension it also means 'first,' 'whole,' or 'same.'
日 — pictograph of the sun
日 is a pictograph of the sun, originally drawn as a circle with a dot or stroke inside representing the sun's substance. Over time the round shape was squared off into the modern rectangular form. It means 'sun' and by extension 'day'.