Usually used when walking on the street and seeing someone you think is gay and wanting to transmit the message to your friends without other people, strangers, knowing.
It started from the following: "What can't you say really fast like gaygaygay....uhm...bigmac bigmac..."
2. A(boy): "I use both shampoo and conditioner that's made specially for blonde hair". B: "LOL....Big Mac!!!" (used as a joke ...in case the boy is just metrosexual)