Regular

Hi,

I'm confused by the following sentence from "Catch-22":
Yossarian says:
"I've got a slight pain in my liver and I haven't been the most regular of fellows, I guess, but all in all I must admit that I feel pretty good"

What could he mean by not being "the most regular of fellows"? Regular in what? Or, is there some special meaning to the word "regular"?

Thanks in advance!!

Regular as in being able to go to the toilet to draw mud on a regular basis.

...as opposed to being constipated.