A noun in The Etymologies of the 1930s glossed “ravine, cleft, gulf” derived from the root ᴹ√YAG “yawn, gape” from primitive ᴹ✶yagwē (Ety/YAG).
Neo-Sindarin: I’d avoid this word for purposes of Neo-Sindarin, as it is easily confused with N. iau “corn” and has a number of other better-known alternatives like S. iâ “gulf” and S. cirith “ravine, cleft”.
A noun in The Etymologies of the 1930s glossed “corn” derived from the root ᴹ√YAB “fruit” (Ety/YAB). It had the rather unusual plural form iui, which seems to be the result of au becoming ui via i-intrusion in Noldorin of the 1930s and 40s (PE22/40): yābī > ı̯auvi > ı̯uiw(i) > iui. In Sindarin of the 1950s and 60s the result was instead oe, so the Sindarin plural of this word would likely be ✱ioew. As for the gloss “corn”, it was probably intended to have its British meaning as a general term for any cereal crop like wheat or oats.