to accumulate over a long period of time
日 · 4 strokes
Pictograph of the sun.
积 · 10 strokes
Phonosemantic compound. 禾 represents the meaning and 只 represents the sound. Simplified form of 積.
月 · 4 strokes
Pictograph of the moon.
累 · 11 strokes
Simplified form of 纍. 田 is shorthand for the phonetic component 畾. 糸 represents the meaning "wind around, bind". The meaning later expanded to "tired".
词汇要靠日积月累,不能一蹴而就。
Vocabulary depends on daily accumulation; it can't be done overnight.