未知数
wèi zhī shù · HSK 7
unknown quantity
❓ 未知数 = 未 not-yet + 知 know + 数 number = a not-yet-known number
The algebra x: a quantity nobody knows yet. 未 (not yet) + 知 (to know) literally stacks into 未知 unknown, then 数 (number) pins it to math. Same wèi as 未来 future, the not-yet-arrived.
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.
数 — 婁 (lóu, phonetic) + 攵 (action radical)
数 is a phono-semantic compound. The right side 攵 (a hand holding a stick) suggests an action, originally tallying or counting by striking marks. The left side 婁 supplies the sound. Over time the meaning broadened from 'to count' to 'number' and 'several'.