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1.
Tier 15 is an extended reference from "Adventure Time" where it refers to love. For example, tier 1 would be just eye contact, tier 7 would be holding hands, tier 15 would be, well, a blowjob.
Oh dude, he's at tier 15 with her!
by P-Bubs September 17, 2013
 
2.
The highest point in a couples relationship. Tier one would be holding hands then sex would only be at tier 7. No one ever goes to tier 15. Its too risky and dangerous. Someone will end up hurt.
Tier four is alright but never go to tier 15!!!!
by tableraccoon August 08, 2014