the act of of sliding your erect and throbbing member in between the buttox of a willing (or unwilling) female. This act is commonly used as a variant of the "titty fuck" and the "buttsex" methods of getting off. The BBM (as is commonly called) can be used when Anal sex is unwanted or flat out denied by the female (as the penis doesn't in fact penetrate the sphincter, it just slides between the buttcheeks).
Great for a first date when she "doesn't do anal".
"hey hussy! can I stick it in your butt? no? how bout a Boston Bun Missile?"