A sting (or stinger) is:

1. Any sharp organ of offense and defense, especially when connected with a poison gland, and adapted to inflict a wound by piercing; as the caudal sting of a scorpion. The sting of a bee or wasp is a modified ovipositor. The caudal sting, or spine, of a sting ray is a modified dorsal fin ray. The term is sometimes applied to the fang of a snake.

2. A sharp-pointed hollow hair seated on a gland which secrets an acrid fluid, as in nettles. The points of these hairs usually break off in the wound, and the acrid fluid is pressed into it.

"Sting" also refers to the wound caused by a stinger, and used as a verb "to sting" is to inflict such a wound.

Source: OPTED, Project Gutenberg and the 1913 edition of Webster's Unabridged Dictionary.