Early 20th century. Apparently from coot + -y, in allusion to the bird's reputation for being heavily infested with parasites; compare as lousy as a coot.
Definition of cooty in US English:
cooty
adjectiveˈkuːti
Infested with lice.
Origin
Early 20th century. Apparently from coot + -y, in allusion to the bird's reputation for being heavily infested with parasites; compare as lousy as a coot.