Examples from the Web for adjectival
It is an adjectival form, like Datis, and means probably "pleasant, agreeable."
The article is omitted before a noun modified by an adjectival phrase.Heath's Modern Language Series: The Spanish American Reader|Ernesto Nelson
They are essentially adjectives, and follow the adjectival rules as to the formation of the plural and the accusative.
The participle has been called an adjectival verbal; the gerund may be called a noun verbal.An English Grammar|W. M. Baskervill and J. W. Sewell
She would remain standing a little stiffly in the scullery refusing to assist him to the adjectival towel he sought.The New Machiavelli|Herbert George Wells