A noun in the Qenya Lexicon of the 1910s glossed “bubble”, an elaboration of pumpo “bubble; bladder; blister” (QL/75). In the contemporaneous Poetic and Mythological Words of Eldarissa the word pumpole was glossed “bubble (a merboy)” (PME/75). It isn’t clear whether “a merboy” indicates that it was also the name of a character Tolkien intended to introduce (such as ᴱQ. Nardi “a flower fairy”, a name appearing in QL but nowhere else), or whether pumpole was term for a kind of sea-fairy, a masculine equivalent of ᴱQ. oaris “mermaid” or ᴱQ. wingild- “[sea] nymph”. I think the second scenario is more likely.