From Old English 'gierd' (rod, measuring rod) — originally a stick's length, unrelated to 'yard' (enclosed space).
A unit of length equal to three feet (36 inches, approximately 0.914 meters).
From Old English 'gierd' or 'gerd' (rod, stick, staff, measuring rod), from Proto-Germanic '*gazdaz' (rod, stick), from PIE '*gʰasdʰ-' (rod, staff). The original meaning was a straight stick or rod, and the measurement was defined as the length of such a rod. It is unrelated to 'yard' meaning an enclosed area (which comes from Old English 'geard,' meaning 'enclosure,' from a different PIE root). Henry I of England is traditionally said to have defined the yard as the distance from his nose
There are two completely unrelated English words spelled 'yard.' The measurement comes from Old English 'gierd' (a rod or stick). The enclosed area ('backyard') comes from Old English 'geard' (an enclosure), which is related to 'garden,' 'garth,' and — remarkably — 'Asgard' and 'Midgard' from Norse mythology. The two words have been confused for over a thousand years