Nullo is a colloquial term for any cisgender man who voluntarily gets his external genitalia removed. The term refers to the way this extreme body modification nullifies the genitals that nullos were born with.
Some nullos also have their nipples removed.
Nullos often prefer this descriptive term because it sets them apart from transgender people, who may elect to remove their external genitalia to look more like the gender they identify with. As nullos are cisgender, this is not their goal. While there is little research into nullos, anecdotal evidence suggests they are mostly homosexual or asexual males.
A nullo may also be called a smoothie or muscunull.