Fath"om (fă&thlig;"ŭm), n. [OE. fadme, faðme, AS. fæðm fathom, the embracing arms; akin to OS. faðmos the outstretched arms, D. vadem, vaam, fathom, OHG. fadom, fadum, G. faden fathom, thread, Icel. faðmr fathom, Sw. famn, Dan. favn; cf. Gr. &?;&?;&?;&?;&?;&?;&?;&?;&?;&?; to spread out, &?;&?;&?;&?;&?;&?;&?; outspread, flat, L. patere to lie open, extend. Cf. Patent, Petal.] 1. A measure of length, containing six feet; the space to which a man can extend his arms; -- used chiefly in measuring cables, cordage, and the depth of navigable water by soundings.

2. The measure or extant of one's capacity; depth, as of intellect; profundity; reach; penetration. [R.]

Another of his fathom they have none
To lead their business.

Fath"om, v. t. [imp. & p. p. Fathomed (?); p. pr. & vb. n. Fathoming.] 1. To encompass with the arms extended or encircling; to measure by throwing the arms about; to span. [Obs.] Purchas.

2. To measure by a sounding line; especially, to sound the depth of; to penetrate, measure, and comprehend; to get to the bottom of. Dryden.

The page of life that was spread out before me seemed dull and commonplace, only because I had not fathomed its deeper import.

• (nautical) A unit of length equal to six feet, used mainly for measuring depths in seas or oceans.
  
• To manage to comprehend (a problem, etc.)
       I can't fathom what this does.