I'm not familiar with the expression "waiter's tongue," googling it didn't bring up any examples but did retrieve a few interesting excerpts of erotica describing waiters highly talented in the nonverbal uses of their tongue.
Michael Smith wrote:
On Sunday 18 February 2007 20:29, Arash wrote:
> And there are language savants who can be fluent in several languages yet
> having trouble with simple tasks with dressing themselves.
I've known people like this. There's a wonderfully dismissive old WASP term for it -- "waiter's tongue."