A nod of the head similiar to the nod of acknoweldgement but can be used between any two men, not just friends. It is an unspoken greeting in which only another man nod is an appropriate response. Signifies acceptence of presence. Verbally may be interpretted as a grunt. Universal language with no ehtnic restrictions.
Bob was walking down a street in a rough neighborhood as he approached a scary looking bigger man. Thankfully the the bigger man gave Bob a man nod, which Bob returned and continued on his way.