Term usually used/heard in combat. Means an individual or persons is extremly vulnerable, possibly out in the open and or helpess in their current position.
"Our men have no cover in that open field. They are sitting ducks to the enemy!"
by UglyMug November 05, 2004
A sex position in which the woman is sitting on a dildo or a erected penis.
My girlfriend loves Sitting Duck.
by SittingDuck June 29, 2009