While crouched over your partner and receiving a hummer
you defecate on your partner's neck, hence a brown bow-tie. Note that the person delivering the brown bow-tie must be facing his partner's head, lest he deliver a brown sombrero
"While Sarah was giving me a hummer last night, those refried beans kicked in and I couldn't help but give her a brown bow-tie!"
The act of doing a girl in the turd cutter and the proceding to slide your meatpole between her melons resulting in what appears to be a "brown bowtie".
Jen had to shower after I gave her a gnarly brown bowtie because her chest smelt like shit.