Meaning

noun a portable balance consisting of a pivoted bar with arms of unequal length

originFrom steel + yard (“enclosed area, rod”). The sense of “place” is a calque of Dutch staalhof or Middle Low German Stalhof (Modern Dutch staal (“steel”) + hof (“court, yard”)). The sense of “balance” is presumably from the sense of “place”, originally “steelyard beam”, mixed with additional sense of “rod” (hence “rod for weighing steel”), replacing earlier Latin statera.

Etymology from Wiktionary, CC BY-SA 3.0