This term is used quite often by players of the online game Counter-Strike, as the game currently must be played on a PC, and 'deagle' is more readily typed than 'Desert Eagle'.

Also, in Counter-Strike, this term is used as a transitive verb (similarly to pistol), indicating that one has taken damage from or been killed by means of a Desert Eagle:

<ueni> Ouch, I just got deagled.

Note that none of this has anything to do with beagles.