A (Noldorin-only?) root in The Etymologies of the 1930s glossed “hole, gap, passage” with the derivative dîn “opening, gap, pass in mountains” (< ✱dēnǝ) which is an element in a couple names from The Etymologies but appearing nowhere else (Ety/DEN).
noun. hole, gap
- ᴹ√GAS “yawn, gape” ✧ Ety/GAS
root. hole, gap, passage
- N. dîn “opening, gap, pass in mountains” ✧ Ety/DEN