Wurdahuzdą: Proto-Germanic dictionary

suffix adj a-stem like English “-est”; forms superlative adjectives, chiefly from regular a-stem adjectives (cf. the comparative suffix *-ōzô) From Proto-Germanic *-ōzô, remodeled after the inherited pair *-izô : *-istaz. The deeper model is Proto-Indo-European *-is-to-, consisting of the zero grade *-is- of the primary comparative/elative suffix *-yos- ~ *-is- plus an old ordinal/superlative dental element *-tó- Fulk 2018: 218–20 Meier-Brügger 2003: 219–22 Ringe 2017: 316–18. In the comparative suffixes, PIE posttonic *s regularly became Pgmc *z by Verner’s Law, giving *-izô and *-ōzô, while the *s remained voiceless before *t in the superlatives *-istaz and *-ōstaz. The *-ō of *-ōstaz is specifically Germanic. Fulk 2018: 218 follows the traditional explanation that *-ōzô arose when *-iz was added to adverbs in *-ō, later spreading from adverbs to adjectives, but notes that it is uncertain; Ringe 2017: 317 is more cautious, stating that the exact preform is not recoverable, but that the suffix contracted with a preceding vowel and then spread by morphological remodeling. As a result, the suffix *-ōstaz has no exact non-Germanic cognate; the IE comparanda belong to the inherited *-is-to- superlative type. Note that *-istaz was the usual superlative suffix for i-, u-, and ja-stem adjectives and was also used with some a/ō-stems, while *-ōstaz was only used with a/ō-stems. In the Northwest Germanic branches *-ōzô/*-ōstaz became much more productive, while *-izô/*-istaz often survived only as relics Fulk 2018: 218–20 Ringe 2017: 317.