bezeg probably originally meant "blown-up, spongy branch", referring to a tree with spongy branches. In particular, elderberry pith is made of a light, spongy substance. Proto-Slavic *bъzъ (together with *bъzgъ, *bъzdъ, and *buzъ) does not have a reliable etymological basis. If the variant *bъzdъ is primary, it can perhaps be formed as a compound with the second Indo-European term, *-sdos (“branch”) (Slovene veja, Greek ὄζος (ózos), German Ast), the first term containing the Indo-European root *bʰew- (“to blow, to swell up”) in different degrees of vowel gradation.
This noun needs an inflection-table template.