grow mouldy
殕 centers on organic decay through fungal growth: something becoming mouldy or rotting.
殕 combines 歹 (death/decay) with 咅, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to decay.
The left side 歹 suggests death or decay, and the right side 咅 looks like something standing and speaking. Imagine a forgotten object left standing so long that it begins to grow mouldy and decay.
For フ, picture mould growing on food: the fuzzy mould makes a soft 'foo' sound as it spreads. Foo -> フ.