The story
From sort (noun) + of. The adverbial use is derived from the use of the term as an adjective in phrases such as “a sort of [something]”.
Etymology from Wiktionary, CC BY-SA 4.0.
sort of
Today it means: Preceded by a (and sometimes also followed by a): of a person, thing, etc.: similar but not identical to another person, thing, etc. Full definition →