足し算
たしざん · tashizan · JLPT N1 · noun
addition
足 — top 口 (mouth/enclosure) + bottom 止 (foot)
Originally a pictograph of a leg and foot, with the upper square representing the knee or shin and the lower part 止 depicting the foot itself. It primarily means 'foot' and by extension 'sufficient' or 'enough,' a meaning shift likely from the idea of standing firmly or having a solid footing.
算 — bamboo 竹 + abacus 具 (eye 目 + two hands 廾)
Originally depicted counting with bamboo rods on an abacus-like device. The top 竹 (bamboo) refers to the bamboo counting sticks used in ancient Chinese arithmetic, while the bottom shows hands manipulating them. The meaning broadened from 'to count' to 'to calculate' or 'to plan'.