*-ô^²

Probably from a fossilized Proto-Indo-European thematic ablative singular case form used adverbially. The prehistory of the ending is disputed. Ringe (2017: 168) derives the Proto-Germanic trimoric adverbial ending *-ô from Core PIE *-e-ad, itself an innovation built from the thematic locative singular *-e plus the particle or postposition *ad (compare Dunkel’s (2014: 180) *é-ad formation). A more traditional analysis, followed by (Fulk 2018: 85–86), reconstructs the thematic ablative as*-o-h₁ed, yielding pre-Germanic *-ōd and then PGmc adverbial *-ô(t). The same ending is preserved with a clearer ablatival sense in Gothic 𐌸𐌰𐌸𐍂𐍉 (þaþrō) (Ringe 2017: 168, 225). In adverbs derived from adjectives, the inherited ablatival value was generalized to a manner-adverbial function. Compare Proto-East Baltic *-ā, reflected in the thematic genitive singular Lithuanian -o and Latvian -a, which Ringe (2017: 168) connects with the same older *-e-ad formation.