Middle Primitive Elvish
den
root. hole, gap, passage
pen(ed)
root. *hillside, slope
tundu
noun. hole
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
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).