'knock-kneed' originates from English; it is a compound of 'knock' + 'kneed' (knee + -ed), where 'knock' originally meant 'to strike' or 'hit' and 'knee' referred to the joint of the leg.

'knock-kneed' developed as a descriptive compound in modern English by combining the verb 'knock' with an adjectival past-participial form of 'knee' (kneed) to describe knees that 'knock' together; it follows the English pattern of forming adjectives by linking an action with a body-part term.

Initially it described the literal action of knees striking or knocking; over time it became established as the term for the anatomical/medical appearance or condition (genu valgum) of inward-turning knees.