Definitions of sitter:

noun: an organism (person or animal) that sits
noun: Dutch astronomer who calculated the size of the universe and suggested that it is expanding (1872-1934)
noun: a person who poses for a painter or sculptor
noun: a person engaged to care for children when the parents are not home
noun: a domestic hen ready to brood
name: A surname (very rare: popularity rank in the U.S.: #29638)