when a group of Mac fanboys sit around in a circle rubbin each other's Macbooks. Usually "oh my god!" is replaced by "oh steve!" very similar to a circle jerk.
when someone buys a macbook, and the others flock around to open and close the macbook over and over and talk about the genius of steve jobs, until they explode into a burst of selfpleasure, hence Mac'ing off
when an apple customer gets excited about his new mac, and just can't contain himself... talk about an upgrade from a floppy drive to a hard disk. just make sure you keep the "sticky keys" option turned off.
i walked into the apple store the other day and saw this group of nerdy guys in the corner mac'ing off to the new macbook pro...
The act of talking about your Mac computer so joyously it seems like you get the same amount of joy from jacking off.
Meghan - Did you hear, James got a Mac Book and an I-touch?
Tom - Yes, I heard. James was Macing off in my ear for the better part of an hour about how awesome his new Mac is.