This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
酛 combines 酉 (sake jar) with 元 (origin). The exact historical development is uncertain, but it likely relates to the base or origin of sake brewing.
The left side is a sake jar 酉, and the right side is origin 元. Picture the original starter mash used to begin sake brewing.
For さけのもと, think of the sake jar 酉 as the origin 元 of sake: sake no moto (さけのもと), the base of sake.