A measure of how many chocolate confections one can secrete under one's foreskin without causing either a) extreme pain; or b) flesh to tear. An average is three. The maximum is six.
Beginners should start with Tic-tacs before moving onto M&Ms. Shoving a bag of Galaxy Minstrels up your foreskin is advanced stuff.
And remember to perform your stretches first.
I was admiring my physique in Asda when I noticed a patch of blood on the front of my lycra shorts. My foreskin had only gone and ripped again hadn't it?