The compound "weak-kneed" is a transparent English formation from two native Germanic elements. The first element, "weak," originates from the Old English wāc, conveying the sense of being pliant or lacking in force. The second element combines "knee," from Old English cnēow, with the adjectival suffix -ed, creating a descriptor for the physical state of one's knees. The term's literal meaning, describing physically unstable knees, provided a vivid metaphor for a lack of firmness in character or resolve. By the early 19th century, this metaphorical extension was fully established, depicting someone who is irresolute or easily frightened, as if their legs were physically giving way.

Structure: weak (lacking strength) + kneed (having knees of a specified kind) - weak: From Old English wāc (pliant, yielding, weak), of Germanic origin. Functions as an adjective describing a lack of strength. - kneed: From Old English cnēow (knee), of Germanic origin, with the suffix -ed giving the meaning "having (a specified kind of) knees". Functions as an adjective-forming suffix attached to a noun.