1. A measure of length containing six feet, the space to which a man may extend his arms; used chiefly at sea for measuring cables, cordage, and the depth of the sea is sounding by a line and lead.
2. Reach; penetration; depth of thought or contrivance.
FATH'OM, verb transitive
1. To encompass with the arms extended or encircling.
2. To reach; to master; to comprehend.
Leave to fathom such high points as these.
3. To reach in depth; to sound; to try the depth.
Our depths who fathoms.
4. To penetrate; to find the bottom or extent. I cannot fathom his design.