Flag is used for denoting "any green and coarse herbage". Water rushes and reeds which grew in marshy regions are believed to be the plants called flag. The word has an Egyptian origin. Genesis 41: 2, 18 has this plant translated as "Meadow". Exodus 2: 3, 5 and Isa. 19: 6 likely are reference to "weeds of any kind".
Source: Smith's Bible Dictionary
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