From Old Norse 'veikr' (pliant), from PIE *weik- (to bend) — weakness as yielding, not lacking strength.
Lacking physical strength or energy; liable to break or give way under pressure; lacking power, influence, or ability.
From Old Norse 'veikr' meaning 'pliant, flexible, weak,' from Proto-Germanic *waikwaz, from PIE *weik- meaning 'to bend, to wind, to turn.' The original sense was 'bendable, yielding' — something weak was something that gave way when pressed, that bent rather than held firm. The word replaced the native Old English 'wāc' (soft, pliant, weak), which comes
'Weak' and 'wicker' share the same PIE root *weik- (to bend) — wicker is woven from pliant, bendable branches. The native Old English word 'wāc' (weak, soft), from the same Proto-Germanic root, was pushed out by the Norse form 'veikr' during the Viking Age, a case of one Germanic cousin replacing another.