An abbreviation of the classic cliche "great minds think alike".
Used as a reply to any situation when you find a connection between your brain and someone else's.
Comes standard with a pretty egotistic tone; implying that your brain is somehow awesomified because an idea you had was just beat into existence a couple 'seconds earlier. Also implies that the one who says it is slightly slower with their thoughts or self-confidence than the one who it was said to.
Great minds son...