This comes from Proto-Indo-European *h₃r̥dʰ-wó-s, also the root of Latin arduus, “steep” and delightfully of Greek ὀρθός, “straight/upright”, root of orthodox, orthogonal, orthodontics etc.
Also Sanskrit ऊर्ध्व (ūrdhvá) “high” and Ossetian уырды́г (wyrdýg) “upright”. (I love the chance to use Ossetian! Nothing in Farsi or Dari, or in any Germanic language for that matter).
A closely related Indo-European root *h₃r̥dʰ-ō-s gives us a high plant in Latin arbor/arbōs, “tree”.
The second element of “Ardennes” is thought to be the Celtic root *windos “white/fair”, which becomes Welsh gwyn (feminine gwen, as in the white enchantress, Gwenhwyfar/Guinevere) and Irish fionn (Fiona, or the fair field Finaghy where I grew up). Not totally convinced, myself!
Going back, the PIE root *h₃erdʰ- comes from the verb stem *h₃er- which also gives us the Latin verb orior “to rise”, from which we get origin, orient and adore; also Greek : όρος “mountain” and ἔρις “quarrel”, which brings us to Slavic рать/рат, “war”.
If it’s war it must be serious, so we get the Germanic root *ernustuz, Dutch ernst, German Ernst, English earnest and the name Ernest. But now I am out of time.
That’s kept me going through the Ardennes. Next time you are passing, look at the fair heights, and ponder the linguistic millennia.