A plum bed (Devonshire). A soft bed, in which the down lies light.
The dough plums well (Devonshire). Rises well, and will not be heavy. The cake is nice and plum (Devonshire). Light. (Plump, swelled out.) He is worth a plum. The Spanish pluma means both plumage and wealth. Hence tiene pluma (he has feathered his nest). We arbitrarily place this desideratum at 50,000 has got only half a plum. “Either a plum or a plumstone”- i.e. “Aut Casar aut nullus.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894