When someone defecates grub size pieces of fecal matter into another persons tub. Similar to an upper decker, but more visibly noticeable. The fecal matter resembles a grub, slowly on its way down the reservoir that is the bath tub drain.
Dude, I totally left a tub grub in my roommate's bathroom. He's going to be so pissed if he ever finds out it's mine.