This unglossed root appears only as an element in the name Azrubêl “Friend of the Sea, Sea-lover” (SD/305). It has a sense similar to that of ✶Ad. √ZIR “love, desire”. Perhap it is distinct from ✶Ad. √ZIR in that it has more to do with friendship than desire. It could be distantly related to the elvish root √MEL, perhaps ✶mel- > ✶mbel- > ✶bel- in the Avari tongue that influenced Primitive Adûnaic, but this is purely speculative.
This root also contradicts statements by Tolkien elsewhere that Primitive Adûnaic only had the vowels a, i and u. Perhaps it should be *BIL, as with ✶Ad. √BITH “say” the primitive root of Ad. bêth “expression, saying, word”.