Definition from Wiktionary, the free dictionary
sitter (plural sitters)
- Someone who sits e.g. for a portrait.
- One employed to watch or tend something; the general form of babysitter, housesitter, petsitter, etc.
- It's always such a pain to get a sitter on short notice.
- A broody hen.
- (soccer and snooker) A really easy scoring chance.
- How could he miss that? It was an absolute sitter!
See also 
- Present tense of sitte
- present tense of sitta.