at one’s back—
1. said of smb. pursuing or chasing a person:
- He hopped on the bike and tore out of the lot, as though the hounds of hell were at his back.
2. said of smb. giving support or protection to a person:
- He has the head of the Department at his back.
on one’s back—
1. said of smb. causing trouble to a person:
- He wants to know who’s going to be Sheriff. That’s the person who’s going to be on his back.
2. said of smb. being a burden to a person (causing expense, etc.):
- We’re the backbone of the country, but you see, Bob, there’s too many people on our backs.
Note: The expression does not correlate in meaning with the phrase on one’s shoulders—being smb.’s responsibility:
- I’d like to get the decision made as early as possible because I don’t want to have it on my shoulders for the whole season.