To be past one’s prime. The analogy to a climber who has reached the top and is now descending has been transferred to the decline of aging since the mid-twentieth century, principally in America. Mary Roberts Rinehart used it in The Pool (1952): “The flawless skin goes, the lovely eyes fade, and she knows she is over the hill.” However, in U.S. military slang during World War II, going over the hill meant going AWOL (absent without leave).