From what I found in the etymological dictionaries, it appears that *stublь and its derivatives descend from proto-Indo-European *(s)tew- (“to hit, to push, to stroke”) extended with -b-/-bʰ-. All sources which I found ultimately qoute Pokorny. Other proto-Slavic terms of this family are likely Proto-Slavic *stьvolъ (“trunk”), Proto-Slavic *studъ (“cold”), and Proto-Slavic *stuga (“cavity, spout”) (the last one is the origin of Old East Slavic стугна (stugna), Bulgarian стуглица (stuglica, “spout within a tree”) (dialectic) and is possibly related to Russian стыгнуть (stygnutʹ, “to cool off”), Polish stygnąć (“to cool”)). Apparently the semantic history of the term went from tool for pushing/hitting > erect, strict, hard > trunk/spout/cold. The problem is that no source clarifies whether the coda is pIE *-b- or pIE *-bʰ-, so I'm reluctant to add it. Bezimenen (talk) 16:29, 31 December 2018 (UTC)