Proboscis is the scientific term for certain mammals' noses, especially long and flexible ones such as an elephant's trunk or a tapir's snout. If someone calls your nose a proboscis, they mean you've got a pretty big 'un.
Proboscis comes from the Greek word for an elephant's trunk, proboskis, which translates as "means for taking food." Not to get too technical, but flies and other insects also have a proboscis, though if you ever end up on "Jeopardy!" it's worth knowing that they are not in fact true noses but actually part of their mouths. So there.
