It's a blowjob, but not just any typical blowjob. The main
difference is
that the blowjob is given by two people instead of just one. The two people acts like the two slices of bread while the reciever's cock is like the meat
sandwiched between the two slices of bread. Thus, the name "cockmeat sandwich" and always involve at least 3 people.
From 2008 movie
Harold & Kumar Escape from
Guantanamo Bay.
This term was used when Big Bob (a guard in
Guantanamo Bay) asked Harold & Kumar to get hungry
real fast and eat his cockmeat sandwich with extra mayo.