A noun in the Gnomish Lexicon of the 1910s glossed “a wave, billow” (GL/68). Given the latter gloss it seems to apply to wind as well as water.
Gnomish
bil
noun. bird
bilin(c)
noun. small bird, sparrow
biltha-
verb. to flutter, flit
sôr
noun. wave, billow
mictha-
verb. to kiss, bill
aigli
noun. bird
cailtha-
verb. to kiss
cailthi
noun. kiss
felcha-
verb. to equal
felectha-
verb. to make equal, equalize
felegra-
verb. to equal
felegrin
adjective. equable
feltha-
verb. to resemble, seem like; to seem likely, be probable
felthadrin
adjective. probable
A word for “bird” in the Gnomish Lexicon of the 1910s appearing next to G. bilinc “sparrow”, but this word was deleted and the gloss for bilinc was expanded to “a small bird, especially sparrow” (GL/22-23). The form bil appeared in a couple other places in the Gnomish Lexicon (GL/23, 31), but seems to represent a “root” rather than a word. Tolkien indicated bil was derived from ᴱ✶du̯il (GL/31), but the exact mechanism whereby du̯- became b- isn’t clear, but a similar change is seen in 1920s ᴱ✶du̯ag- > ᴱT. baga- “beat” and ᴱ✶tu̯ak- > ᴱQ. pak- “apply, attach” (PE14/66).