I've heard this a couple times now and most recently in a movie "钢的琴". It's an idiom that means "cut me some slack"; "let me off the hook". I couldn't make sense of the Baidu encyclopedia so I'm asking here.
It should be 放我一马. 马 here refers to the rider rather than the horse itself. It's from 三国演义.
English version of the story (wikipedia)