The acceptance of a male into a group of males. This usually involves the group getting together and boozing (preferably keeping female contact to a minimum). The warmers must ensure the warmer gets to a relatively high level of intoxication so as to ensure a proper temperature is gained.
Despite it's connertations a lad warming in no way involves and explicit contact between attendees.
"hey dude are you going round the girl's to pre drink"
"fuck that man it's Johnti's lad warming, we're going to get him so drunk he spews in the neighbours garden"