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1.
(v) Aviator slang meaning to fixate yourself on something to the point where you loose track of your surroundings. Literally to boresight means to line up the axis of a gun with its sights or to line up other avionics such as a HUD or an ILS. The term refers to the fact that the pilot fixates himself on his sight trying to shoot down an enemy and does not notice what is going on around him, and can have potentally devastating consequeses if he fails to recognize threats around him.

In a general sense, to tell someone that they are boresighting means that they are too focused on something and need to look around and see what is going on.
P1: He's in my sights... 300 yards... he's not getting away

P2: Dont Boresight theres a bogey at 6

P1: Oooh craaaaaaaap

or in a civilian environment

PI: lemme get this one done... im nearly there
P2: Dont boresight you have 34 more to finish
by Beanerous Brokus I January 14, 2007