先月
せんげつ · sengetsu · JLPT N5 · noun
calendar month
📅 先月 = previous (先) + month (月)
先 means ahead or earlier, as in 先生 (one born earlier) and 先週 (last week). 月 is moon or month, read げつ here as in 月曜日 (Monday). Earlier + month = the month before this one. Swap 先 for 来 and you get 来月, next month.
先 — 𠂉 (person variant) on top + 儿 (legs) below
先 originally depicted a person with a foot or step shown above the legs 儿, suggesting someone walking ahead. From this image of going before others came the meanings 'first,' 'earlier,' and 'ahead.' The modern shape preserves the 'legs' radical underneath.
月 — pictograph of crescent moon
月 is a pictograph of a crescent moon, chosen over a full circle to distinguish it from 日 (sun). The inner strokes originally depicted the moon's shadow or markings. As a radical it often relates to the moon, time, or (via a lookalike form of 肉 'flesh') body parts.