* Un-: This prefix means "not" or "the opposite of".
* Fathom: This word originally referred to the length of a human arm, and later evolved to mean a unit of length used to measure the depth of water.
Therefore, unfathomable literally means "not able to be measured or understood by a fathom".
This word has been in use since the late 16th century, and its meaning has remained largely consistent over time.