(Also in form granny's bonnets, with singular concord) any of various plants whose flowers are thought to resemble granny bonnets, especially a columbine (genus Aquilegia).
Origin
Early 20th century.
Definition of granny's bonnet in US English:
granny's bonnet
noun
English Regional
(Also in form granny's bonnets, with singular concord) any of various plants whose flowers are thought to resemble granny bonnets, especially a columbine (genus Aquilegia).