means A point of vulnerability or fault — a lack of strength, whether in body, character, structure, or resolve; also a soft spot, the thing you can't resist.

from From Old Norse 'veikr,' meaning pliant or bendable, which slipped into Old English as 'wac' — soft, yielding, easily moved. The same root gives us 'weak' itself and is a cousin of 'wicker,' that bendy basket-work. The '-ness' is the old English ending that turns a quality into a thing you can name. So at heart, weakness is simply the state of bending where you'd hoped to hold firm.