To ask - What is a flirt mug??

[Florence1960]

I’m seeing these advertised on Etsy and EBay. I have some lovely mugs I want to sell and they appear to be flirt mugs - but I don’t know what makes them such! Does anyone know?
It isn’t linked to the make or designer -Alex Clarke
Doesn’t seem to be the shape - mine flare at the top but I’ve see. Plain up and down ones
Not the pattern I don’t think - so what is it??

Anything to do with a penis beaker?

It’s the shape of the mug but seems to be specific to the designer. If you Google it, one of the results is Alex Clark’s mugs and it says “shape: flirt”

That really made us laugh!
I really did google it but everything seemed to be a flirt mug. Thank you, that’s really helpful.