In the TV series M*A*S*H, who was the first person to call Margaret "Hot Lips"?
She actually got the nickname in the M*A*S*H movie. She's having sex with Frank Burns, unaware that the camp PA system is on. She's overheard begging him to kiss her "hot lips." After that, the name stuck. And it was transferred into the TV show as well.
As for who first called her that, in the movie, it was Duke Forrest(source). The link is to the movie script. Her name (Margaret) in the movie was only ever said once, by Frank. As for the TV show, without watching the Pilot, I'm not certain. But I'd presume Hawkeye or Trapper.
Major Houlihan first got the nickname "Hot Lips" in the M*A*S*H movie, (1970):
While Henry is away visiting General Hammond at the 325th Evac Hospital, Trapper leads the camp in a general abandonment of regulations, and wild partying ensues. Burns and Houlihan are appalled and write a report to the General. They also give in to their repressed passions and engage in a sexual encounter. Their tryst is broadcast over the public address system; everyone hears Houlihan telling Burns to "Kiss my hot lips!", earning her the nickname "Hot Lips". Hawkeye quietly taunts Burns about the encounter, goading Burns to attack him. Burns is sedated, restrained and shipped stateside.
So she had the nickname prior to the start of the series. Is that what you wanted to know, or are you asking which character in the series first said the words "Hot Lips"?