Safety Pin. A pin bent back on itself, the bend forming a spring, and having the point fitting into a kind of sheath, so that it will not prick the wearer while in use, usually used for fastening infants' clothing. Safety pins range in size from 1, 1 1/2 , 2, 2 1/2 up to 4, from small to large in the order named. Also called nursery pins.