This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development is uncertain. The character combines 魚 (fish) with 免, but the original semantic or phonetic role of 免 in this context is unclear.
The left side is 魚 (fish), and the right side is 免. Since the meaning is unclear, you might imagine a fish that is exempt (免) from being caught, but this is purely a mnemonic device.
For ベン, picture a bent fishing rod catching a fish (魚) while the fisherman is exempt (免) from paying a fee: bent -> ベン.