"Did you mean to produce a Stinging Hex?"
. . . "No..."
. . . "I thought not..."
-- Severus Snape and Harry during Occlumency (OP24)
The Stinging Hex is a fairly low-powered hex that causes a stinging pain in the victim and leaves a burn mark (OP24).
References from the canon
When under assault by Snape's Legilimency, Harry found it almost impossible to resist the intrusion until Snape happened upon the memory of Harry's kiss with Cho Chang. The resistence he felt to Snape viewing this memory translated into a Stinging Hex that broke Snape's spell. Harry had not consciously decided to perform that spell, so it seems unlikely that that he actually spoke any words. It would seem instead that this hex was purely an extension of his intention to fight Snape off (OP24).
The Stinging Jinx is a similar spell, perhaps a stronger version of the hex, which not only causes pain but also causes the victim to swell up (DH23).