A furious chase between Portree Seeker Dougal McBride and Arrows Seeker Gregory Cotton ended in a skybound scuffle, and while Cotton emerged with the Snitch, his lap of honour was brutally curtailed as McBride turned his head into a cabbage. ...Referee Josiah Plunkett attempted to remonstrate with McBride, who insisted that Cotton had put a Jelly-Fingers curse on him as they both swooped down on the Snitch. Both Seekers were searched and discovered to have wands illegally upon their persons. An enquiry is expected.
-- Daily Prophet, 1st June, 1999 (DP3)
Presumably makes the target’s fingers like jelly, so that they cannot grasp anything properly.
References from the canon
- After a June 1999 Portree/Arrows game, the losing Seeker accused his opposite number of putting this curse on him as they both closed in on the Snitch (DP3).