Pregnant; knocked up. Means literally that "one" kid is "on the way" (will be born soon, and will consume every moment of your spare time with its endless whining). Not a good thing if you have a girlfriend/wife who is not the mother of the baby.
"Just when I thought I said all I could say, my chick on the side says she's got one on the way..." -Usher, "Confessions (Part II)"
Jerry: "Shit man, did you hear? I got one on the way."
Pete: "Awesome. It's good you stopped shooting blanks and finally knocked your wife up."
Jerry: "No...that's the problem. My wife ain't the baby's mama...your 16-year-old daughter is."
Pete: "That dirty slut. I need to get her on the pill."