Okay, I missed Flag Day by a couple of of days.
Flag (n/v) - The noun (cloth ensign) comes from the verb (to flap loosely), or the other way around. Authorities disagree which came first, the rag or the flap.
Authorities also disagree on the source word. It is possibly Old Norse (Norwegian has its word flagg), possibly Germanic (the Dutch word vlag is obviously related), possibly English through and through. My best guess is the word is Norwegian in origin, spread by Viking raiders.