To keel over means to suddenly fall down, usually because of sickness, exhaustion, or surprise. When someone keels over, they topple sideways or forward, like a tree falling in the forest. You might see someone keel over from heat exhaustion on a hot day, or a character in a movie might keel over in shock after hearing surprising news.

The phrase comes from sailing. The keel is the long, heavy piece that runs along the bottom of a boat, keeping it stable in the water. When a boat keels over, it tips completely on its side, which is dramatic and dangerous. Just like a boat losing its balance, a person who keels over loses theirs.

People often use this phrase with a touch of drama or humor. A runner might joke, “I almost keeled over after that race,” or someone might say, “Don't work so hard you keel over!” The phrase suggests a sudden, complete collapse rather than a gentle, controlled lowering to the ground. When you feel so tired you could keel over, you're so exhausted you feel like you might just topple right where you stand.