"one who commits robbery by breaking into a house," 1540s, shortened from Anglo-Latin burglator (late 13c.), earlier burgator, from Medieval Latin burgator "burglar," from burgare "to break open, commit burglary," from Latin burgus "fortress, castle," a Germanic loan-word akin to borough.
The unetymological -l- is perhaps from influence of Latin latro "thief" (see larceny). Middle English had burgur (c. 1200), from Old French burgeor, burgur, also housbreker (c. 1400). Burglar-alarm is by 1840.