fathom - definition and synonyms

verb [transitive] 

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/ˈfæðəm/
present tense
I/you/we/theyfathom
he/she/itfathoms
present participlefathoming
past tensefathomed
past participlefathomed
present tense
I/you/we/theyfathom out
he/she/itfathoms out
present participlefathoming out
past tensefathomed out
past participlefathomed out

or fathom out British 

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  1. to understand something complicated or mysterious

    For some reason she couldn’t fathom, he seemed angry.