Like the more notable, strap on, the dick on a belt allows a person without a penis to engage in pleasuring their partner(s) with a large, hard, cylindrical object.
The dick on a belt can refer to a homemade strap on as well, which is constructed of your everyday dildo/vibrator, a belt & perhaps some duct tape.
Jennycame over last night and fucked me with her dick on a belt.
When a female has her seat-belt on and it crosses between her boobs making them look bigger in size which would later create a boner for the guy sitting in the car next to her looking at them.
"Hey Chubb's look at blonde in the car next to us. She has some awesome seat-belt titty's. I'd sure like to stick my cock between them"