When a girl forces herself upon guy in such a manner that it becomes an unstopable force of nature that no one can resist, hard as they may try. Much like someone who tries to swim away from a tsunami.
Often used to reference the song Cyclone "Tryin' to show me her tsunami/ slunami"
Girl 1: I can believe Anne made out with Trevor last night
Girl 2: He was helpless to resist, she showed him her slunami