-undus

Possibly from Proto-Italic *-ontnos, syncopated from *-ontinos, from Proto-Indo-European *-ont- (participial suffix) + *-i- (abstract noun-suffix) + *-nós (adjective-forming suffix), or from Proto-Indo-European *-ótr̥. The -undus suffix, first associated with intransitive deponent verbs as sequor (secundus), orior (oriundus), lābor (lābundus), and other relics, then transited from a derivative to an inflectional category, and the gerundive assumed its canonical shape and function.

The sense of necessity (“needing to be read,” etc.) was a pragmatic outgrowth of the more basic meaning: a book that is readable or “disposed” to be read is one that probably should be read. The replacement of -undus by -endus was a late development under the influence of the present participle; older Latin still has vertundus, legundus, etc.

The derivation from gerundive to the later gerund could then be as easy as an abstraction from “ad librum legendum” (“to the book to be read”) to “ad legendum” (“in order to read”).[1] Compare -bundus and -cundus.

• (Classical) IPA^(key): /ˈun.dus/, [ˈʊn̪d̪ʊs̠]
• (modern Italianate Ecclesiastical) IPA^(key): /ˈun.dus/, [ˈun̪d̪us]

-undus (feminine -unda, neuter -undum); first/second-declension suffix

1. Derives adjective nouns from verbs, similar to present participles, but without the present time restriction

   sequor (“follow”) → secundus (“following”)

   orior (“rise”) → oriundus (“rising, originating”)

   lābor (“glide, slide, fall”) → lābundus (“falling”)

   rotō (“turn around, roll”) → rotundus (“rolling”)

First/second-declension adjective.