to party like there's no tomorrow. Live like today's your last. In the 1983 Prince hit "1999" he refers to "2000-0-0 party over, oops - out of time" so tonight he "parties like it's 1999". This became a catch phrase in the American lexicon.
It means to throw a wild party. The expression is dated and comes from an old Prince song that obviously was written before 1999. The idea was the last party of the 20th Century. Some, in those days, also believed that the end of the century might coincide with the end of the world, so party like it’s the last party of your life.