The term is used dismissively and describes something real. A holder with no defined reason and too much size will sell on the first serious move against them, regardless of what happens afterwards.
Weakness is a function of position and preparation, not character. The same trader is weak-handed at ten times normal size and perfectly steady at normal size. That is the useful reading: if you keep getting shaken out, the fix is usually sizing and a written invalidation rather than resolve. See paper-hands and strong-hands.
Related: paper-hands, strong-hands, shakeout, position-sizing