Muʿaṭṭal is a passive participle associated here with being unadorned or lacking ornaments. Wa-lā bi-muʿaṭṭal denies that state, implying that the neck is not left without adornment.
The verse uses two consecutive negations: laysa bi-fāḥish rules out ungainly excess, while wa-lā bi-muʿaṭṭal rules out absence of jewelry. Together, they imply grace combined with ornament.
This illustrates affirmation through negating opposites. Rather than simply declaring the neck “well-proportioned and adorned,” the poet excludes two disfavored alternatives.