fathom

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A unit of length equal to 6 feet (1.83 meters), used principally in the measurement and specification of marine depths.
2. v. To penetrate to the meaning or nature of; comprehend.
3. v. To determine the depth of; sound.

Century Dictionary and Cyclopedia

1. n. Originally, the space to which a man may extend his arms; specifically, a measure of length containing 6 feet: used chiefly in nautical and mining measurements.
2. n. Hence Mental reach or scope; penetration; the extent of capacity; depth of thought or contrivance.
3. To encompass with the arms extended or encircling.
4. To reach in depth by measurement in fathoms; sound; try the depth of; penetrate to or find the bottom or extent of.
5. Hence To penetrate with the mind; comprehend.

Wiktionary

1. n. Grasp, envelopment, control.
2. n. A measure of length corresponding to the outstretched arms, standardised to six feet, now used mainly for measuring depths in seas or oceans.
3. v. To encircle with outstretched arms, especially to take a measurement; to embrace.
4. v. To measure the depth of, take a sounding of.
5. v. To get to the bottom of; to manage to comprehend (a problem etc.).

GNU Webster's 1913

1. n. 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. n. The measure or extant of one's capacity; depth, as of intellect; profundity; reach; penetration.
3. v. To encompass with the arms extended or encircling; to measure by throwing the arms about; to span.
4. v. To measure by a sounding line; especially, to sound the depth of; to penetrate, measure, and comprehend; to get to the bottom of.

WordNet 3.0

1. v. measure the depth of (a body of water) with a sounding line
2. n. a linear unit of measurement (equal to 6 feet) for water depth
3. v. come to understand
4. n. (mining) a unit of volume (equal to 6 cubic feet) used in measuring bodies of ore