The "man nod" is an unspoken greeting when you want to acknowledge someones presence, but do not wish to start a conversation with them. The only way to answer a man nod is with another man nod, only verbal part of a man nod is usually a grunt. The man nod is great for being sort of polite even when you dont want to speak to them at all. The man nod can also be done to non men by men, and also used on strangers if your a blokey bloke.
Steve was walking down the street when he saw Fred he knew from uni coming the other way, being a bloke Steve didnt want to bother with being social so he gave Fred a quick man nod, as Fred walked past he returned Steves man nod and carried on walking.
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.