water flag
/ˈwɔːtə ˌflaɡ/noun
Either of two irises growing in streams, pools, and marshes: yellow flag, Iris pseudacorus, of Eurasia, and blue flag, I. versicolor, of North America.
Origin
Late 16th century; earliest use found in Henry Lyte (1529–1607), botanist and antiquary. From water + flag.