According to OLD, the adj. viridis derives from the verb vireo, but nothing is mentioned about the suffix that turns the verb to the adj. Could anyone tell about the suffix that transforms the verb vireo to the adj. viridis ? (I looked up viridis in wiktionary, and like the previous search in OLD, no relevant info was found therein as well.)
The suffix -id- creates an adjective from a second-conjugation stative verb (a verb that describes a state instead of an action). For example:
- cal-eō "to be hot" → cal-id-us "hot"
- torp-eō "to be sluggish" → torp-id-us "torpid"
- liqv-eō "to be flowy" → liqv-id-us "liquid"
Usually the resulting adjective is in the first/second declension, but sometimes it takes the third declension instead. I'm not sure why, but that would be another good question to ask!