What does the idiom over hill mean?

The idiom "over the hill" does not exist. It's likely you're thinking of "over the hill," which is a common idiom meaning:

Past one's prime; aging and no longer youthful or vigorous.

For example:

* "He's feeling a bit over the hill since he turned 60."

* "The car is over the hill and needs to be replaced."

This idiom often carries a negative connotation, suggesting a decline in physical or mental abilities due to age.