What is 78?
A quick and discrete method of informing someone who is about to answer the phone that you're "not here." Traditionally, once you've declared your 78 status (simply by saying the number when the phone rings), the person answering is required to inform anyone asking for you that they've not seen you since 1978. In some cases there is leeway for alternative responses (as when the person on the other end may be familiar with 78) as long as you are protected from having to take the phone. If everyone in a room calls 78, the last person to call it must answer the phone.
An override code is also sometimes used. This is a predetermined year (post-78) that friends can use to override 78 and reach you in an emergency. The correct usage of the override is to tell the person who is answering "You're no help then; I was talkin to him in 83 (or 84, or whatever you've picked)"
Jeremy (being the only one left, answers the phone): Hello? Greggo? I ain't seen Greggo since '78. Sorry.
78 is similar to 69, 89, or 39, but it's hammer rape. the seven represents a hammer, while the 8 is a butt.
Grace: "god! i hate that man!"
Julia: "we should 78 that douche bag!"
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?