Meaning

noun one of four children born at the same time from the same pregnancy

noun a muscle of the thigh that extends the leg

noun (printing) a block of type without a raised letter; used for spacing between words or sentences

originShortened, via clipping or abbreviation, from various English terms. Ultimately related to Latin quattuor (“4”); compare quadri-, quadruple. The typography senses are from the abbreviation quad. for obsolete quadrat; the keyboard command comes from the verb sense.

Etymology from Wiktionary, CC BY-SA 3.0