Other forms: grubbiest; grubbier
Grubby things are really dirty, like your little sister's grubby hands after she's spent an afternoon making mud pies.
Have you ever seen a little kid eat a chocolate ice cream cone on a hot day? Picture the kid's face covered in chocolate and you'll have a good idea of what grubby means! This adjective dates back to the mid-19th century, from grub, which means "insect larva" and which was once colloquially used to mean "dirty child."