Stickum is a spell that magically adheres two objects together. It may or may not be temporary. Stickum is apparently such an simple spell that, once mastered, it can be done easily without the use of a wand. The incantation is probably a play on stick 'em.
Barry used this spell on a first year who was kissing Dorco's ring. Later in the year the first year came off suggesting that the spell was either temporary or someone used another spell to undo it.