I have learned that there is a suffix -cundus, found in words like fecundus, jucundus/jocundus, and rubicundus, which means something like "full of" or "characterized by." It seems to often be mentioned along with participles ending in -ndus and -bundus (such as Allen and Greenough's New Latin Grammar).
Apparently, it is of "disputed origin" (The Foundations of Latin, Philip Baldi 1999) but I'm interested in learning what etymologies have been proposed.
I did find one paper that contains a summary of the current "state of the art": «Une loi phonétique méconnue en latin : la lex- hircus», by Romain Garnier (in French; I couldn't find the publication date but it must be recent since it has citations from 2015).
Garnier says Beneviste (1933) and Szemerényi (1950) proposed derivations from a PIE root *ḱuH- meaning «être gonflé» ("to be swollen, inflated"), followed by a suffix (according to Beneviste, -ond-ó-; according to Szemerényi, -tn-ó-).
De Vaan (2015) is supposed to have proposed that the c is not part of the suffix at all, and that fecundus should be analyzed as fec-undus with the same stem as in the perfect feci.
Garnier himself argues that facundus comes from a form *bʰéh₂-tu- followed by a group of suffixes *-u̯o-dn-ó, which syncopated to *fātu̯ond-o, and then assimilated to *fāqu̯ond-o (which subsequently developed according to generally recognized sound laws).
I don't really know how to evaluate these etymologies. Which one is most widely accepted? Are there any others that I haven't found?