Even longer than honorificabilitudinitas and attested in antiquity is subductisupercilicarptor (24 letters). If you want to compare it to a dative/ablative plural, then in fairness the word form you should compare would of course be subductisupercilicarptoribus (28 letters).
It is a word for an overly discerning critic and may perhaps be translated as raised-eyebrow carver (from subducere = raise [actually a rare meaning], supercilium = eyebrow, carptor = food carver).
OK, it's a bit of a joke word that was never in common use. It occurs in one place only, and then it is not really used either, just quoted.
The word was presumably coined by the somewhat mysterious poet Laevius from the second century BC, of whom only scant fragements have survived, but the actual source is Aulus Gellius: Noctes Atticae, book 19, where A. G. reports on the style of Laevius and his proclivity for coining original expressions. For he had attended a reading of Laevius in the house of one Julius Paulus together with his companion Julius Celsinus, and on their way back the two reminisced on the various “Laevian” coinages they had heard, and pondered which of these they might adopt for their own use. After giving a number of examples, Aulus Gellius concludes:
Cetera [verba] enim, quae uidebantur nimium poetica, ex prosae orationis usu alieniora praetermisimus; ueluti fuit, quod de Nestore ait 'trisaeclisenex' et 'dulciorelocus', item quod <de> tumidis magnisque fluctibus '<fluctibus>' inquit 'multigrumis' et flumina gelu concreta 'tegmine' esse 'onychino' dixit et quae multiplica ludens conposuit, quale illud est, quod uituperones suos 'subductisupercilicarptores' appellauit.
Other words namely, that seemed too poetical and inappropriate for use in prose, we passed over; one example was that he called Nestor “trisaeclisenex” and “dulciorelocus,” likewise that for high-rising and great rivers he spoke of “fluctibus multigrumis,” and said that rivers that are frozen solid are “tegmine onychino,” and what multi-part words he playfully put together, such as that he called his critics “subductisupercilicarptores.”