Donegal district court bordering on the grumpiest (8)

I believe the answer is:

crossest

I'm a little stuck... Click here to teach me more about this clue!

'grumpiest' is the definition.
('cross' can be a synonym of 'grumpy')

I don't understand how the remainder of the clue works.

(Another definition for crossest that I've seen is " most annoyed".)

I've seen this clue in The Irish Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.