The bottom part of a doorway; points at which something begins or changes; minimum levels required for something to occur.
Old English 'þrescold,' possibly from 'þrescan' (to thresh) plus 'wald' (wood), referring to the wooden board where grain was threshed. The metaphorical sense of 'beginning point' developed by the 14th century.
This word beautifully bridges the physical and abstract - from the literal board you step over to enter a house to the metaphorical point where change begins. The agricultural etymology reminds us that many abstract concepts grew from concrete, physical experiences of daily work.
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