The banged up knees children get when playing outside in shorts and dresses that don’t protect their knees from cuts and abrasions. It’s a wonderful phase for I absolving parents of the guilty from inevitable summer injuries.
Example: “How did Leroy cut his knee?” (With the suggested guilt implied of weren’t you watching him and why haven’t you bandaged it yet?)
Parent response: “Oh, he just has summer knees.”
Excellent phrase for free-range parents to know.