For those of you, who like me, couldn't remember what a Tardigrade is, they're microscopic animals that live in water and are also known as Water Bears.

https://en.wikipedia.org/wiki/Tardigrade

All I can think of is that this is some obscure word used to test functionality of their the piece of software that blocks payments based on a keywords contained within. I suppose someone could test out a transaction with obvious blocked keywords like "nazi terrorist child porn" and see if it behaves similarly, but I'm not going to try it.