To burglarize! The bastard verb came out of nowhere about 1871. It served no useful purpose not already served by the derivative forms of "burglar," a fine old 16th-century noun. In those simpler days, a burglar was one who burgled, i.e., one who broke into another's dwelling at nighttime with the intent to commit a felony.
As so often happens, the apostles of nicey-nicey came along. They felt that "to burgle" had a vulgar ring to it. It rhymed with "gurgle." It had the sound of mouthwash swishing. "Dayclossay!" they cried. Thus they tacked on an "-ize" suffix...
What he seems to be saying here - it's not very clear - is that burgle is the oldest form, and burglar is a noun formed from it by use of an agentive suffix - like tester from test or runner from run. Z.D. Smith points out in the comments to yesterday's post that this is not true. burglar dates from the 15th century, and burgle is a much more recent (19th century) backformation (says the Oxford Dictionary of English Etymology).
Kilpatrick's second point is that burglarize was coined because burgle sounded too vulgar. In fact, burgle and burglarize appeared at the same time, and I'm not aware of any evidence that burglarize was motivated by some sort of language hygiene attitude.
In summary, the "derivative forms of 'burglar'", specifically burgle, that Kilpatrick assumes existed and served a useful purpose in the 16th century, did not exist. It was a jocular backformation some 400 years later. And burglarize appeared at the same time as burgle.
But enuf of Kilpatrick. burglar is from "legal Anglo-Norman" burgler, from a base *burg- (says the ODEE). According to the AHD, Old French burg "borough" (without the asterisk) is from Latin burgus "fortified town", of Germanic origin. The Proto-Germanic word is *ƀerǥan "to protect", from Proto-Indo-European *bʰerǵʰ- "high, with derivatives referring to hills and hill-forts".
Proto-Germanic *ƀerǥan combined with *friþu- "shelter" (from PIE *priH- "love", as in free) to form something like *ƀerǥ-friþu-. This was borrowed into Old French as berfrei "watchtower" - that is, "place of safety". This was borrowed into Middle English as berfrey "A movable tower used in sieges; the chamber in a bell tower where the bells are hung". An Old French variant spelled with <l>, belfroi, influenced an English form spelled with <l>: belfrei, bellefrei. The association of towers with bells no doubt influenced this spelling as well. The spelling with <l> eventually prevailed: belfry.