I like this answer. Mine would've been "spirit of nature incarnate" or similar, but this captures why I think that.

Tom Bombadil is trustworthy in that he was understood to be incorruptible by the ring. However, he wasn't a trustworthy holder of the ring because he'd probably lose it because he didn't feel the gravity of it. Tom Bombadil is good and trustworthy, but ultimately uncontrollable.