必要
ひつよう · hitsuyou · JLPT N4 · na-adj
necessary
必要 = must (必) + need (要): a must-need is necessary
必 (hitsu) means certainly or without fail, as in 必ず. 要 (you) means need or essential, as in 重要 (important). Put together: something you certainly need, so it is necessary.
必 — heart 心 + slash 丿
Originally depicted a handle or marker driven through something, suggesting a fixed standard. The form was later reanalyzed as 心 (heart) with a stroke through it, reinforcing the sense of inner certainty. It came to mean 'must' or 'certainly'.
要 — west 覀 (top) + woman 女 (bottom)
Originally a pictograph of a woman with hands on her waist, meaning 'waist' (the original form of 腰). The top 覀 is a stylized depiction of the waist area, not the character for 'west'. Over time it was borrowed for the meaning 'to want, important', so 腰 (with 月 flesh radical) was created for the original 'waist' sense.