Q From Pierre Garon, Canada: I read in the newspaper the following sentence: ‘And if that is not enough to harsh his mellow, his pal wades in about making the rent and getting his act together’. I make a wild guess that harsh one’s mellow might mean to ‘harry one some more’ but it might be a case where your lights are needed.
A You’re pretty much there. Don’t harsh my mellow is American slang, meaning variously and roughly “don’t treat me badly”, “don’t get on my nerves”, “don’t make life difficult for me”, roughly the same as buzzkill in phrases like “don’t be such a buzzkill”. It’s a development of US campus slang, in which in the 1980s harsh became a verb in the sense of “to mistreat”, “to be very unfair to”.
The longer expression seems to have originally been West Coast drug and hacker slang of the middle 1990s. It became more widely known in 1997 when it turned up in The Online Adventures of Ozzie the Elf on ABC television. When Ozzie is criticised by an elf in Santa’s workshop, he says, “Don’t harsh my mellow”. Since then, as you’ve discovered, it has begun to appear from time to time in mainstream newspapers and magazines; I’ve seen it in Time magazine and also in the issue of Fortune for March 2003: “That guy really harshes my mellow, and I don’t appreciate it”. However, it has not yet become, and may never become, a common slang term in the USA.
Mellow here was presumably at first a reference to that gentle high one gets during a drug trip but may well now have been modified to refer to any comfortable feeling of being at ease. To harsh it is to introduce a jarring or discordant note, usually because you’re being criticised or leaned on by some figure of authority.