Mêd is a Doriathrin adjective for “wet” derived from primitive ᴹ✶mizdā, appearing as an element in the mountain name Dolmed “Wet Head” (Ety/MIZD). First the short [i] became [e] preceding the final [a], then the [e] lengthened due to the vocalization of [z] before voiced stops. It is unclear, though, whether the vowel lengthened directly (as suggested by Helge Fauskanger, AL-Doriathrin/méd), or whether it first became the diphthong [ei] after which [ei] became [ē] (the theory used here).
Conceptual Development: After abandoning the Ilkorin language, Tolkien retained the name Dolmed. It is possible Tolkien reconceived of this word as Sindarin, but if so, its Sindarin form should perhaps be *mêdh, not mêd, since voiced stops became spirants after vowels in Sindarin. In Silmarillion map revisions from the 1950s-60s, Tolkien did write Dolmeð (WJ/183 section F14), but he never made the corresponding change in the narratives.
Neo-Sindarin: For the purposes of Neo-Sindarin writing, it would be better to use one of the other attested Sindarin words for “wet”, such as nîn¹.