Gratuitous, outrageous or unusual use of the tongue while kissing, often exhibited by actors in
pornographic movies.
It can range from letting the tips of the tongues flicker over each other while the lips remain several inches apart,
to something that looks like slugs mating.
Porno tongue does not require two tongues - one can apply porno tongue to the ear, nipples, genitals etc.
There is also 'solo' porno tongue, in which a person (usually male) flicks and waves their tongue around when nobody is anywhere near it, usually while slapping their own arse or tweaking their own nipples - kinda like playing air guitar, but much, much sadder.