Grit

Grit--it's a word that swings both ways, functioning  both as a noun and a verb.  "To grit one's teeth" is to press them hard against each other--usually in frustration or anger.  And to have grit is to have courage, spunk.

What I didn't know is that the word, coming from  Middle English, is derived from a word for sand, or small hard particles.  And it's used in geology to describe just that.

Makes sense--grits (a common breakfast cereal in the South) is just that!  Gritty....