Crossword clues for fthm fthm A linear unit of measurement (equal to 6 feet) for water depth (mining) a unit of volume (equal to 6 cubic feet) used in measuring bodies of ore WordNet fthm n. a linear unit of measurement (equal to 6 feet) for water depth [syn: fathom] (mining) a unit of volume (equal to 6 cubic feet) used in measuring bodies of ore [syn: fathom]