All etymological dictionaries includung Wiktionary and De Vaan’s Etymological Dictionary state the ultimate origin of the Latin adjective idoneus (‘suitable’; ‘sufficient’) is unknown.
I was wondering if anybody had a theory on the origin of this word; I would be glad if someone could shed light on this problem.
Also, De Vaan says:
The morphology can be interpreted as an adj. in -neus to *idō (‘there’?) or *ido-, or as an adj. in -eus to *idōn.