Synonyms for Sitting duck:

n.

gull, chump, sap, patsy, dupe, sitting target, mug. butt (noun)
sap, chump, easy mark, patsy.
fall guy (noun)
patsy, dupe.
patsy (noun)
gull.
sitting duck (noun)
easy mark.

Other synonyms:

sitting target, chump, dupe, patsy. sap, gull. mug. Other relevant words:
easy mark, chump, patsy, mug, dupe, sap, gull.