This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
燓 combines 棥 (a fence-like structure of crossed branches) with 火 (fire). The exact historical development is uncertain, but it likely relates to burning or a fire-related action involving wood.
The top 棥 looks like a tangle of branches, and 火 is fire underneath. Picture a bonfire of crossed sticks burning.
For フン, imagine the fun of a campfire: fun -> フン, as the crossed branches burn brightly.