Meaning of “fathom” - Learner’s Dictionary

fathom

verb [ T ] uk us /ˈfæðəm/ ( also UK fathom out)

to be able to understand something after thinking about it a lot:

[ + question word ] No one could fathom why she had left so early.

(Definition of “fathom verb” from the Cambridge Learner’s Dictionary © Cambridge University Press)