fathom

(noun) a linear unit of measurement (equal to 6 feet) for water depth. Synonyms: fthm.

(noun) (mining) a unit of volume (equal to 6 cubic feet) used in measuring bodies of ore. Synonyms: fthm.

(verb) come to understand. Synonyms: penetrate, bottom.

(verb) measure the depth of (a body of water) with a sounding line. Synonyms: sound.