The phrase “low talker” was popularized by Seinfeld's “The Puffy Shirt” for someone who speaks so quietly that others struggle to hear them.
In the episode, Jerry meets Kramer's girlfriend Leslie, a fashion designer whose unusually soft voice makes conversation difficult. Because Jerry cannot clearly hear what she is saying, he mistakenly agrees to wear a flamboyant, pirate-like shirt during an appearance on The Today Show. The misunderstanding drives the episode's central joke.
“The Puffy Shirt” originally aired on September 23, 1993, as the second episode of Seinfeld's fifth season and the 66th episode overall. The story was inspired by Larry David's experience talking with a real-life low talker, though the Seinfeld plot turns that everyday annoyance into an escalating public disaster.
The phrase is sometimes confused with “close talker,” another Seinfeld expression associated with someone who stands uncomfortably near others while speaking. That term comes from a different episode, “The Raincoats,” so “low talker” is the canonical answer here.