Drunk and high. When a person drinks (one slice of bread) in combination with smoking (the other slice of bread), they become the ham sandwich.
Unsuspecting College Freshman1: "Hey man, lets get really high tonight."
More knowledgeable upperclassman: "Awesome, lets smoke your weed."
*Proceed to get really high*
Unsuspecting College Freshman2: "Hey guys, I bought beer from this homeless guy."
UCF1: "I can't turn down free beer, lets get drunk!"
MKU: "I'll sit this one out."
UCF1+2: Puke everywhere and die.
MKU: "Man, right before they died, they were totally ham sandwiched."