I climax where you begin
Kendrick is already at his Peak when you are just gettin Started. (Still with the toilet imagery)
Another possibility is that the term “climax” is used sexually. “Where you begin” could mean in a person’s mother’s vagina, thus Kendrick could be saying that he climaxes in your mother.
To help improve the meaning of these lyrics, visit "Rigamortus" by Kendrick Lamar Lyrics and leave a comment on the lyrics box