Definition of blue flag
: a blue-flowered iris; especially : a common iris (Iris versicolor) chiefly of the eastern U.S. with a root formerly used medicinally
First Known Use of blue flag
Medical Definition of blue flag
1: either of two common blue-flowered irises (Iris versicolor and I. virginica) of the eastern and central United States with rhizomes formerly used medicinally as a cathartic and emetic
2: the dried rhizome of a blue flag—called also blue flag root
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