The Brief: The phrase "white whale" refers to an obsession that is dangerous to an individual's safety and/or health.

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The expression “white  whale” is used to refer to an activity or thing  an individual obsesses over and/or is constantly chasing to the point that it becomes dangerous for their health or safety.

The white whale is an individual’s ultimate goal, according to Urban Dictionary.

It is derived from the story of Moby Dick. Captain Ahab was obsessed with hunting and killing the white whale, Moby Dick.