1782, earlier cullibility (1728), probably from gull (n.2) "dupe, sucker" + -ability.
gules
gulf
gull
Gullah
gullet
gullibility
gullible
Gulliver
gully
gulp
gum
)window.location='https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=byronic106@yahoo.com&lc=US&item_name=Donation+to +Help+Keep+Etymonline+Free+and+Open&no_note=0&cn=&curency_code=USD&bn=PP-DonationsBF:btn_donateCC_LG.gif:NonHosted'){kind=link}