An idea scrum is a figure of speech to describe "putting it to the team" or the fierce debate amongst teams members on a certain topic or attempting to win over an arguement. It is based upon the scrum in rugby union or rugby league.
In an idea scrum, the hooker (manager) places the ball (idea/topic for debate) into the scrum (the various team members) who attempt to win possession (fiercely debate the topic and try to persuade the other team members) before the ball is won (a resolution is met)
Gaz: "Hey Gaz, do you reckon Raz has an enormous head?"
Gaz 2:"Dunno gaz, lets idea scrum it"
Derick: "Hey Gary, how was your day at work?"
Gary: "Pretty shit to be honest Derick, I spent half the day sending emails and the other half idea scrumming shit ideas with my team"