78 refers to something being cool or awesome.
sometimes referred to as 'page 78'. When used as 'page 78' it refers to calling someone nerdy.
Note: When speaking of 78 one usually makes the gesture of pushing up their glasses with their index finger. This occurs whether the person wears glasses or not.
Dude! that's 78!
Guy 1: Hey the answer to that is on page___
Guy 2: what did you like read the whole textbook or something? where's the next answer? page 78?