means Covered in small sharp points, or — of a person or mood — easily irritated and quick to take offense.

from From 'prickle,' the Old English 'pricel,' a little goading point, kin to 'prick.' The '-y' tacks on the sense 'full of,' so 'prickly' literally means 'full of tiny stabs' — a meaning that drifted naturally from thorns and hedgehogs to touchy tempers, since both make you wary of getting too close.