A term used to describe a failure state of lithium-ion batteries, such as those found in smartphones and laptops. A battery can become a spicy pillow due to multiple factors, including (but not limited to) the battery's age, physical damage to the battery, and/or an error in manufacturing or installing the battery. In this state of failure, the battery has entered "thermal runaway": unwanted chemical reactions inside the battery cause heat and poisonous gases to build up inside it.

The "pillow": the build-up of gases inside the battery causes it to swell, resembling a pillow. This often happens slowly over time. The swelling is an obvious sign that the battery has failed, or is about to fail. The pressure build-up may in turn make the battery's device bulge and then come apart, e.g. a phone's screen cracking and/or separating from the phone body.

The "spicy": the swollen battery is now a fire and/or explosion hazard. Improperly-contained spicy pillows have destroyed vast swathes of electronic equipment, set houses ablaze, and sent people to the emergency room with third-degree burns, often due to the phones in their pockets.

When you eat dirt for the first time and leave your family to build a shack in the woods

A battery that has puffed up to an expanded state, popularized by r/spicypillows on reddit.