Son: Look, Dad, I can balance on the railing!
Dad: You’re cruisin’ for a bruisin’, son. Get down from there before you hurt yourself.

“Cruise” is a type of travel. Boats cruise, especially cruise ships. Some cars have cruise control, a feature that keeps their speed constant without the driver’s input. “Bruise” is a mark on a person’s skin caused by hitting or being hit by something hard. This phrase
warns you that your current course is likely to get you bruised, or injured.