1
general::
An easy target, as in If you park in front of a fire hydrant, you're a sitting duck for a ticket. This term alludes to the ease with which a hunter can shoot a duck that remains in one spot, in contrast to one in flight. [First half of 1900s]
American Heritage Idioms
2
general::
Phrase(s): *a sitting duck
Fig. someone or something vulnerable to attack, physical or verbal. (Alludes to a duck floating on the water, not suspecting that it is the object of a hunter or predator. *Typically: be ~; like ~; looking like ~.) • You look like a sitting duck out there. Get in here where the enemy cannot fire at you. • The senator was a sitting duck because of his unpopular position on school reform.
McGrawhill's American Idioms And Phrasal Verbs