Nictitate didn't just happen in the blink of an eye; it developed over time as an alteration of the older verb nictate, which also means "to wink." Both verbs trace to the Latin word for winking, nictare. The addition of the extra syllable was apparently influenced by Latin verbs ending in -itare, such as palpitare and agitare (which gave us palpitate and agitate, respectively). Today, nictitate has a special use in the animal world. Since the early 18th century, scientists have used nictitating membrane to describe the so-called "third eyelid": the thin, usually transparent membrane in the eyes of birds, fishes, and other vertebrates that helps keep the eyeball moist and clean.