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.
by DoppleGanger14 June 19, 2011
When you sit in dirty bath water and never use soap so there is a ring of dirt around your tub from not washing
I went to mias house and used the bathroom only to see her shower curtain open and her tub to have tub grub
by ViperzZ1706 September 6, 2020