“…the term halakhah pesukah need not be considered the opposite of minhaga. It may simply mean a rule that cannot be questioned or supported on the basis of conventional talmudic dialectics”

Perhaps it can be argued that the ḥumra of the daughters of Israel is termed a halakhah pesukah not because it is “a ruling that all agree to,” as explained by Zimmerman (this is an anachronistic explanation based on the later use of the verb pasak to refer to halakhic rulings), but because it is rooted in custom, rather than in texts or formal discussion, and therefore can engender no further analysis. This explanation is borne out by BT Nidah 66a, the halakhic source of the ḥumra of the daughters of Israel: When Rav Papa challenges a legal ruling of Rava’s on the basis of the ḥumra, Rava fends him off by arguing: “I’m talking about a legal prohibition; you’re talking about custom; [in matters of custom,] wherever they were strict, they were strict.” Seen in this light, the term halakhah pesukah need not be considered the opposite of minhaga. It may simply mean a rule that cannot be questioned or supported on the basis of conventional talmudic dialectics.

Moshe Benovitz, “A Lifetime Companion to the Laws of Jewish Family Life and Man and Woman: Guidance for Newlyweds (review),” Nashim: A Journal of Jewish Women’s Studies & Gender Issues vol. 12, no. 1 (2006), 327, n. 18.