I thought that was from the other JFK.
There was a story (no idea if true) that when John Effin' Kerry took office as Lt Governor of Mass., he took to signing all his correspondence "JFK", ordered paper with a "JFK" watermark for his letters, and had his office workers refer to him in the same fashion. It didn't take long for this to get back to Teddy. So Teddy went to JFK's office and told him that there was only one JFK, and if Lt Gov Kerry wanted to continue in politics, he would stop immediately or find out how the fast a career could end.