The verb ᴹQ. tumba- appeared in a marginal note to a rejected page of verbal roots in the Quenya Verbal System (QVS) from 1948, with the somewhat difficult to read gloss “to cast down (into a pit[?] etc.[?])” (PE22/127). It was derived from the root ᴹ√TUB meaning “go down, below normal ground-level, esp. to go down (sink, dive) into water”. The other derivatives of this root, adjective tumba “deep” and noun tumbo “deep valley”, continued to appear in Tolkien’s later writings.

Conceptual Development: A possible precursor to this verb was ᴱQ. tum or tub “dive” in Early Qenya Word-lists of the 1920s (PE16/132), which in turn was likely derived from the early root ᴱ√TUM(B)U (QL/95) as suggested by the editors.

Neo-Quenya: I would retain ᴺQ. tumba- for purposes of Neo-Quenya as a weak verb based on the adjective tumba “deep”, with the original sense “endeepen = go (or cause to go) deep”, hence with transitive meaning “cast down (into depths)” but also intransitive “dive”.