A Mennonite Necklace is the accumulation of perspiration that gathers around the collar of a t-shirt. It is important to note that a proper Mennonite Necklace is distinguished by the fact that the shirt has no other sweat stains apart from the collar area.
Iain: Look at Steve, he's a true Mennonite.
Mags: How do you know?
Iain: Look at his Mennonite Necklace... It's not even cold out.