Euphemism for mastubation. Orginated in the Harry Potter fandom. It comes from the fact that to gain entrance into the school kitchens, one must tickle the pair on a painting of fruit guarding the entrance. Hermione Granger learns this when the Weasley twins let the secret slip.
Harry bit his lip, trying not to let a whimper escape as he tickled his pear, fantasizing desperately about Dobby the House-elf.