未必
wèibì · HSK 4
may not
未 = not yet + 必 = must/certain; 'not yet certain' = may not
未 means 'not yet' (the negation in 未来 future). 必 means 'must/certainly'. Together: 'not necessarily certain' — something may or may not be the case.
未 — tree 木 with extra short top stroke
未 originally pictured a tree with luxuriant new branches at the top, suggesting tender shoots not yet matured. From this 'not yet ripe' image it came to mean 'not yet' or 'have not.' It is also the 8th Earthly Branch, used in the traditional sexagenary cycle.
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'.