white whale

Slang for relentlessly or obsessively pursuing an object that's difficult to achieve, as in "She's still doing screenwriting? Yep, that's her white whale." It usually implies a lack of regard for one's own safety and the safety of others, a chase that you'll never win, and some sort of tragic ending.

Historical perspective: It comes from the 1851 novel Moby Dick, by Herman Melville, in which Captain Ahab is obsessed with finding the whale who injured him on a previous voyage.