Popcorn is a colloquial term for XP-valuable monsters of a low level that are easily killed.
Popcorn is usually the result of careful play that takes advantage of the bonus experience gained from killing high-level monsters exclusively. The surplus lower-level monsters can usually be killed in a single strike, incurring no loss of health. With a large popcorn reserve, players can harvest enough popcorn to level up once or even twice in quick succession. This is a common strategy for fighting bosses or high level enemies, often called a 'mid-level ding', where a player depletes their resources before killing enough popcorn to gain a level and receive full health and mana.
Popcorn can also synergize with many gods who grant large amounts of piety for killing enemies. Popcorn can be a reserve of piety as well as XP and even health for characters with life steal
Slowing enemies is an excellent way to eke out extra XP from popcorn enemies. The use of WONAFYT and WEYTWUT can be useful, as can certain items and boons. Using WONAFYT during early exploration create what is known as a 'popcorn bowl': a central reserve of slowed monsters that are always accessible and easily harvested. This is one of the primary uses for WONAFYT and can be a very powerful resource during the late game.