-ibly

• -ably

Euphonic blend of -ible +‎ -ly, due to difficulty of pronouncing *-iblely. Compare same sound change in unrelated feebly, nimbly, and nobly.

-ibly

1. Used to form adverbs corresponding to adjectives that end in -ible.

   audible → audibly

As with -ably, generally only used to form analogs to adjectives ending in -ible, replacing the existing -ible suffix (respectively -able). Rather than a distinct suffix, this can be considered a class of euphonic blends of “-ble” and -ly, as in feebly, nimbly, and nobly, among others.

Further, as with -ible, generally unproductive as an suffix to existing English words – most words ending in -ibly are derived from words ending in -ible borrowed from Latin or Old/Middle French, not formed by suffixing in English. Productive uses in English include digestibly.