Two statues of Gondorian kings just north of Nen Hithoel, translated “Pillars of the Kings” (LotR/392), more literally “(Pair of) Royal Stones” (RC/347, RGEO/67, Let/427). It is combination of the prefix ar(a)- “noble”, the noun gond “stone”, and the class-plural suffix -ath (RC/347).
It is not clear why this noun uses the class-plural. However, in one place Tolkien describes a dual variant argonad using the archaic dual suffix †-ad (Let/427). Perhaps this was the original form of the name, altered to Argonath after the Sindarin dual form was forgotten.
Conceptual Development: In Lord of the Rings drafts from the 1940s, this name first appeared as N. Sern Aran(ath) “King Stones” along with an alternate name N. Sern Ubed “[Stones of] Denial” (WR/98, 132). Later in these drafts it was revised to N. Argonath (TI/366).
arn (prefix “royal”) + gond (“great stone, rock”) + ath (collective plural suffix). Triconsonantal cluster rgn produces rg; in this case original nd doesn’t produce nn before the collective plural suffix ath. #It might be a dialectal form or a mistake on the part of Gondorians who didn't always use Elvish languages correctly (VT42 “Rivers and Beacon-hills”).