Merriam-Webster's Collegiate® Dictionary, 11th Edition
SITTING
I. noun
Date: 13th century
1. the act of one that sits, a single occasion of continuous sitting (as for a portrait or meal),
2. a brooding over eggs for hatching, setting 6, session ,
II. adjective
Date: 15th century
that is setting , occupying a judicial, legislative, or executive seat ; being in office, easily hit or played ,
4. used in or for sitting , performed while sitting