kind of water-beetle cash, then dollars and money generally
蚨 centers on a specific kind of water beetle that was historically associated with money, and by extension came to refer to cash or money in general.
蚨 combines 虫 (insect) with 夫, which likely contributes the sound フ. The character originally referred to a water beetle, and its association with money comes from the ancient Chinese phrase 'fúqián' (蚨錢), where the beetle was thought to return to its mother, symbolizing money that comes back.
An insect (虫) that looks like a man (夫) is a water beetle. In old stories, this beetle's blood was used to make money that would always return, so 蚨 came to mean cash.
For フ, imagine a water beetle that makes a 'foo' sound as it skims across the water: foo -> フ, and that beetle is the money beetle.