An armful is as much as a pair of arms can hold at once. An armful of logs, carry an armful, armfuls of flowers.
It is a measure by the body rather than by any standard, like a handful or a mouthful, which is why it survives in speech and never in a recipe. The plural is armfuls.
1nounarmfulAs much as a pair of arms can hold at once.
“She came in from the garden with an armful of fresh flowers.”
armful is a quantity.
Arm plus -ful, on the pattern of handful and mouthful.
From arm + ful.
Etymology from Wiktionary, CC BY-SA