a less formal way of saying angry or irritated, this phrase 'aggster' expresses a feeling of anger, ironically in a less angry way. it could be angry at a specific event, however irritated could be a better definition of the word. can often be shortened to 'aggy'
adding -ster can also be used on other words such as
chubster (chubby- a person who is chubster)
grottster (describing something as grottster)
person 1, "why are you so aggster today?"
person 2, "ah, claire is really p***ing me off at the moment!"