A constellation of sixties hippie expressions which are still quite current. This is the standard hip youth greeting, as ubiquitous as "What's happening?"
phrase used to ask your ex "what's up" after a while of not talking to them. "que onda perdida" means "what's up lost girl" referring to the fact that your ex is lost without you.
me: qué onda perdida (what's up lost girl)
ex: nada, te extraño😩(nothing, i miss you😩)