Tyvnk is the name given by Rysn to the potted Shin grass which she has been instructed to care for by her babsk, Vstim. The name means 'sullen'. In naming the grass, Rysn acknowledged that her babsk had been right - and that she'd been wrong - about his trade with the Shin. She'd chosen to let the grass be sullen, rather than herself, at being proven so clearly wrong.
Even so, Rysn liked to pretend that that her pot of grass was not stupid, but merely contemplative.