what is a double adjective
I presume it refers to a pair of adjectives used in combination, probably connected with and. For example:
Phoebe was confident and self-assured as she walked into the meeting.
We have many of these that are fixed phrases. For example:
Chicken soup is a tried and true method for treating a cold.
I'm sick and tired of your lame excuses!
Tony was black and blue after the fight at the playground.
Hope this helps.