必需
bìxū · HSK 5
need
必 (must) + 需 (require) = must-require, i.e. need
必 means 'must/certainly' and 需 means 'require/demand'. Together 必需 stacks two near-synonyms to mean 'essential/needed'. Compare 必须 (must do) — same 必, different second char for the verb sense.
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'.
需 — rain 雨 + and 而
Originally a pictograph showing a person standing in the rain, meaning 'to wait' for the rain to stop. The bottom component 而 (originally depicting a beard or a standing figure) was later reanalyzed phonetically. The meaning shifted from 'wait' to 'need' or 'require,' while the older 'wait' sense was taken over by 須.