The adjective delch “dense” appeared in notes probably from the mid-1960s as a derivative of primitive ✶delkā based on the root √DEL “thick, dense” (PE17/17). The note had an “X” next to it indicating rejection, followed by another note where Tolkien said “the proposed √DEL ‘thick, dense’ will not, however, do: it does not provide suitable Quenya or Sindarin forms”.

Neo-Sindarin: Since we have no later words for “dense”, I retain ᴺS. delch “dense” for purposes of Neo-Sindarin. However, to better distinguish it from other √DEL roots I assume it is derived from a Neo-Root ᴺ√NDEL so that delch had ancient primitive initial nd- [*ndelkā], based on the Quenya verb form Q. nelya- “thicken, congeal” [< *ndelyā?] appearing in the same note; see that Quenya entry for discussion.