A roomful is enough people or things to fill one whole room. A roomful of people, a roomful of furniture, addressed a roomful.
It is one of a family of container measures — a handful, a mouthful, a houseful — and like all of them the plural is roomfuls rather than roomsful. It is deliberately vague and does the work of saying a great many without counting.
1nounroomfulA quantity of people or things large enough to fill one whole room.
“She walked into a roomful of strangers and didn't know where to stand.”
roomful is a quantity.
Room plus -ful, on the same pattern as handful and mouthful.
Middle English
roum
English
room
English, today
roomful
Built from
From room + ful. room comes from Middle English roum. From Middle English roum (“room, space”), from Old English rūm (“room, space”), from Proto-West Germanic rūm (“room”), from Proto-Germanic rūmą (“room”), from Proto-Indo-European (H)rewH- (“to root; to rip, tear”), from Hrew- (“to tear out, open”). 2
Etymology from Wiktionary, CC BY-SA