fathom

Grasp, envelopment, control.

(now usually nautical) An English unit of length for water depth notionally based upon the width of grown man's outstretched arms but standardized as 6 feet (about 1.8 m).

Various similar units in other systems.

Depth of insight, mental reach or scope.

To encircle with outstretched arms, especially to take a measurement; to embrace.

To measure the depth of, take a sounding of.

To get to the bottom of; to manage to comprehend; understand (a problem etc.).

“I can't for the life of me fathom what this means.”