A word glossed “tunnel, small grot” derived from the root √s-rot (PM/365 note #56), appearing in 1959 notes discussing the origin of the name of Felagund, which in that document based on a Khuzdul name. The English word “grot” is an archaic form of “grotto”, referring to a cave.
Conceptual Development: There are a number of similar “cave” words in the Qenya Lexicon of the 1910s derived from ᴱ√ROTO “hollow”, including ᴱQ. orot, ᴱQ. rótele, and ᴱQ. rotl, the last of these glossed “cave, hollow” with a deleted variant {rotta >> rotto} (QL/71, 80). This last word also appeared as ᴱQ. rotl “grot” in the Poetic and Mythological Words of Eldarissa (PME/80). ᴹQ. rotto “cave, tunnel” appeared under the root ᴹ√ROT “bore, tunnel”, a late entry to The Etymologies of the 1930s (EtyAC/ROT). This root reappeared as the basis for “cave” words a number of places in later writings (PE17/183; VT39/9; WJ/414-415), but the only later mention of rotto is the aforementioned 1959 note on Felagund, given above.
rotto noun "cave, tunnel" (VT46:12), "a small grot or tunnel" (PM:365)