Is the word "so" an adjective? ex: That dress was sooooooooooooooo fabulous.
It is an adverb. See explanation.
An adjective modifies a noun.
I read an interesting book yesterday.
In this sentence word interesting modifies the noun book, so it is an adjective.
An adverb can modify either a verb or an adjective.
an adverb modifying a verb: I went quickly to the kitchen. - Adverb quickly modifies the verb went.
an adverb modifying an adjective: The book was so boring, that I fell asleep just after a few pages. - the adverb so modifies the adjective boring.
In the given sentence word so modifies the adjective fabulous, therfore it is an adverb.