Iarwain
S. [jˈɑrwɑjn] adj. old-young (that is old, but yet still very vigorous) ◇ LotR/II:II, RC/128 ◈ Elvish name of Tom Bombadil, rendered as "oldest" in LotR, which made some people interpret this word as a superlative form (of iaur). However this theory was later controversed. While no other superlatives in *-wain are known, einior, attested in PM/358 (where it is rendered as "eldest"), makes a much better candidate for a superlative construct (conceivably built from the intensive prefix an- and iaur shortened in final position). Moreover, comparing iarwain to narwain, Bertrand Bellet noted in Lambengolmor/642 that iarwain could as well be interpreted as an old compound construct, of the primitive *jâra (Q. yára, S. iaur) and *winjâ (Q. vinya, S. gwain). He thus suggested that iarwain could actually mean "old-new". This analysis was afterwards confirmed by RC/128, quoting a (yet) unpublished draft letter from Tolkien dated from 1968, which states: "Iarwain = old-young, presumably as far as anybody remembered, he had always looked much the same, old but very vigorous" ◇ iaur+*gwain
Source
Alphabetical Sindarin-English dictionary