Why is the ninth month called September?
1853 in literal sense of "pertaining to a groove," from groove (n.) + -y (2). Slang sense of "first-rate, excellent" is 1937, American English, from jazz slang phrase in the groove (1932) "performing well (without grandstanding.)" As teen slang for "wonderful," it dates from c.1941; popularized 1960s, out of currency by 1980. Related: Grooviness.