If used in "they went back outside" it is an adverb modifying went.
If used in "he is back outside now" it is a predicate adjective.
Adverb
The words "on the outside" are three different parts of speech. on: preposition the: article outside: noun
It is an adverb modifying the verb (playing).
What's the part of speech of the word outside
An adverb of place.
It may be a noun (the outside of a ball) adjective (outside restroom) adverb (went outside) preposition (outside the house)
The word outward is an adjective. It means to be located towards the outside.
Exterior can be an adjective and a noun. Adjective: Relating to outside parts. Noun: The outside parts.
adjective
Cringe is a verb meaning to shrink back in fear or disgust
The word gate is a noun. It is a door-like structure outside of a house which usually encloses the garden.
Pat is a verb and a noun. Verb: Pat yourself on the back. Noun: You deserve a pat on the back.