420 from the urban dictionary:
http://www.urbandictionary.com/define.php?term=420
the term 420 originated at San Rafael High School, in 1971, among a group of about a dozen pot-smoking wiseacres who called themselves the Waldos, who are now pushing 50. The term was shorthand for the time of day the group would meet, at the campus statue of Louis Pasteur, to smoke pot. Intent on developing their own discreet language, they made 420 code for a time to get high, and its use spread among members of an entire generation.
So there ya go, someone's parents out there invented the term 4/20.
among thr various defintions
Greek:
Same source definition #3:
/noun/(1) a citizen of Greece; (2) a person whose ancestors are from Greece
/adjective/ referring to things or ideas that originated in Grecce/verb/ to butt fuck
http://www.urbandictionary.com/define.php?term=greek