Generally a derogatory term used to describe a male (though it can often work for the appropriate female as well). The term is derived from the straightness of a flag pole, therefore a man who is a flagstaff is extremely straight and shows this through his treatment of girls in his life. An example of this is when a guy ditches his friends to
go watch a chick flick to impress some girl. Similarly a girl could be a flagstaff when she ditches her friends to go watch a sport to which she doesn't even know the rules only to impress some guy.
Whipped is a term that is used similarly, however
whipped generally refers to the dominance of the significant other whereas flagstaff refers to a personality trait possessed by the one who is being
whipped (more specifically, a flagstaff
will be a flagstaff no matter who his significant other is whereas one who is
whipped may only be so because the significant other demands it).