Original line is "
me love you long time" from the Stanley
Kubrick film Full Metal Jacket.
Has evolved over the years but is usually used as a form of acknowledgment instead of saying excellent or cool.
"Hey bro whats going on wanna chill?"
"
Good longtime"
Can also denote the quality of something and longtime vice versa.
A Ferrari is
good longtime but a holden Commodore is longtime.