It's a pet name referencing your significant other/ mate, and holding them at the highest honer possible. Some one you dated in the past couldn't or wouldn't have what it takes to earn this pet name, it is a privilege. Now, someone you marry, or your soul mate can wear this title and they will wear it well.
Matt Damon calls out to Kim K. " Bootykins!!! I wuv ur lil face!" Kim K. would respond as such, "Bootykins? Booty...Booty..Kins? ... I LOVE YOU! My BOOTYKINS!"