Obviously it's a play on the name of a famous muscle car (the Pontiac Bonneville) though it refers to neither muscle nor car, but a very stiff erection that steers its owner wherever it wants to go (to a nice warm parking garage of course).
A term used on 4chan's /lgbt/ board to refer to the information that one will not pass as the gender they identify as on account of their bone structure. Individual bonepills include comparisons between male and female Q angles, skull size measurements, ribcage measurements, and other sexually dimorphic skeletal features.
I was considering facial feminizationsurgery until I took the bonepill and realized that my skull was too large to ever pass as female.
Immediately after the intro of the Victoria Secret Fashion Show featuring Marisa Miller, Alessandra Ambrosio, Heidi Klum and other thorough-bred dime pieces in hardly any clothing, a commercial of an overweight Elvis is aired. Ew...Bonerkill.