The name originated in San Francisco about 1870–72, and began to excite attention elsewhere in the U.S. about 1877, by which time its origin was lost, and many fictitious stories, concocted to account for it, were current in the newspapers. See a selection of these in Manchester (N. H.) N. & Q. Sept. 1883. [OED2]

origin unknown [W3]

Dear Word Detective: I have been told that the word "hoodlum" is "Muldoon" spelled backwards, which some newspaper writer used but his editor changed the N to an H in fear of retribution. What city, what paper and who was Muldoon? -- L.S. Lahrson, via the internet.

You don't happen to have the person who told you that in custody at the moment, do you? No, I suppose not. It's rare that the infernal agents of the Confusion Cabal who spread stories like that hang around to be apprehended. I guess it wouldn't be much of a cabal if they did.

The story you have heard is, to put it delicately, utter bunk. But if it's any consolation, it's genuine antique utter bunk, first recounted in a slightly different form by John Bartlett in his 1877 Dictionary of Americanisms. In Bartlett's account (according to Hugh Rawson's marvelous book "Devious Derivations"), a newspaper editor in San Francisco dubbed a gang of street urchins (run by a thug named Muldoon) "noodlums" in a moment of wit, not fear. A typesetter, however, supposedly misread the "n" as an "h," and "hoodlum" was born. Now, it is true that "hoodlum," meaning "a rowdy or street thug," did first appear in San Francisco around 1870. Unfortunately, there is no evidence that any of the rest of that story ever happened, and Mr. Rawson quite rightly terms this story "noodleheaded" and "one of the most fanciful of all folk etymologies."

That's far from the only silly story you're likely to hear about the origin of "hoodlum," however. Perhaps the most preposterous maintains that "hoodlum" arose simply as a mispronunciation of "hooligan," which would be a good trick, since "hoodlum" is found in print around 1870 and "hooligan" didn't appear until 1898.

Mr. Rawson's best guess as to the true source of "hooligan" is the Bavarian word "Huddellump," meaning "a slovenly person," a theory that fits with the fact that at the time "hoodlum" first appeared, German immigrants were the largest non-English-speaking minority in San Francisco.

Hugh Rawson's excellent "Devious Derivations," by the way, is published by Crown and available in paperback. Buy it -- it's your best defense against the Confusion Cabal.
© The Word Detective

NB: I assume the WD meant "true source" of "hoodlum" in his penultimate paragraph.