This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
膅 combines the flesh radical ⺼ with the phonetic component 唐. The exact historical meaning is uncertain, but it likely referred to a part of the body or a type of flesh.
The flesh radical ⺼ suggests a body part, but without a clear meaning, picture a mysterious lump of flesh labeled with the sound 唐.
For トウ, imagine touching a strange fleshy lump and saying 'toe' in surprise: toe -> トウ, as you poke the unknown body part.