This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
熳 combines 火 (fire) with 曼, which likely contributes the sound. The exact historical development is uncertain.
The fire radical 火 beside 曼 suggests something related to flames or heat, but the exact meaning is uncertain.
For マン, imagine a man standing too close to a fire: man -> マン, and the fire 火 on the left warns of danger.