Most Players Get This On Accident. Norman Reedus' Bladder Gets Full And You Got To Pee. Your Shame Is Marked By A Mushroom Hologram And The Achievement Pops.
If You Stop After Each Delivery To Use The Facilities And Take Showers (To Make Free Grenades) You Won't Get This One. Took Me 70 Hours To Finally Need To Pee Outside.
Side Note -
I Think Drinking The Energy Drinks Also Increases Your Bladder Meter.
Good Luck!