From OE 'fæðm' (outstretched arms) — a body-length depth measure whose verb 'to fathom' extended from sounding seas to grasping ideas.
A unit of length equal to six feet (1.83 meters), used chiefly in measuring the depth of water.
From Old English 'fæðm' (outstretched arms, embrace, bosom, a unit of length), from Proto-Germanic '*faþmaz' (outstretched arms), from PIE '*pet-' (to spread, stretch out). The original meaning was the span of a person's outstretched arms — fingertip to fingertip — which averages about six feet. The verb 'to fathom' (to understand, to get
A fathom is your armspan — the distance from fingertip to fingertip when you stretch your arms wide. Sailors measured depth by dropping a lead-weighted line and hauling it up hand over hand, counting arm-spans. The verb 'to fathom' (to understand deeply) comes from this practice: to fathom something is to sound its