The Dude Nod.
When two guys pass each other...there is 'the dude nod'. Its either a nod down..which seems to be a more respectable nod, almost something like tipping your hat back in the day. Sort of an unsaid 'hello sir'.
Then there's the up nod. Sort of a 'hey...i dont know you...so im keeping my eye on you' kind of nod...the nod you give the guy that you see milling around your girl when she's waiting on you at the bar and you're not around yet.
These nods can be followed by verbal cues that seem to range from a grunt...to 'sup'...and if you're a bigger guy a 'hey boss' kind of response.
Do women have these kind of things too?