A neologism coined by Paul Strack in 2018 specifically for Eldamo, an adjectival form of S. madha “mud”, inspired by G. ûgrin “muddy” the adjectival form of ûg from the 1910s. It would need to be a late Sindarin (re)formation rather than the phonetic evolution from ancient forms.
Sindarin
madha
noun. mud
madhren
adjective. muddy
madha
noun. mud
mud
A word given as maða “mud” in the Outline of Phonology (OP2) of the 1950s, a derivative of ✶mazgō (PE19/101). A similar form N. mada [probably a slip for maða] appeared in The Etymologies of the 1930s with the gloss “soft, pliant, yielding” as a derivative of madhgh < ᴹ√MASAG (EtyAC/MASAG). But this form was rejected and apparently replaced by N. moe “soft, pliant” < ON. maiga (Ety/MASAG). These variations seem to reflect shifting thoughts on the development of zg in Sindarin and Noldorin, either (in the 1930s) zg > i̯g to form diphthongs or (in the 1950s) zg > ðg > -ða.