1/2 pr stone
砆 is a rare kanji referring to a specific type of stone, described as a "1/2 pr stone". Its meaning is unclear and likely a specialized mineralogical term.
砆 combines 石 (stone) with 夫, which likely acts as a phonetic component. The exact historical development is uncertain.
The stone radical 石 beside 夫 suggests a specific kind of stone. Imagine a husband (夫) holding a peculiar stone (石) that is exactly half the weight of a standard stone.
For フ, picture a husband (夫) holding a stone and saying "Foo!" in surprise at its unusual weight: Foo -> フ.