1. When you're trying to take a dump while on the phone, you claim you're "lifting something heavy" in order to explain the grunting.
2. This phrase can also be used as a euphemism for that act in other situations.
1. MAN 1: (grunting)
MAN 2: "Dude, are you shitting while we're on the phone?"
MAN 1: "No, I'm just lifting something heavy."
2. MAN 1: "Where are you going?"
MAN 2: "I'll be back in a few minutes. I have to lift something heavy."