Raucous is an adjective describing something harsh and loud — raucous laughter, a raucous crowd. Ruckus is a noun meaning a noisy disturbance — they caused a ruckus. They sound similar and are different parts of speech, so "a raucous broke out" and "a ruckus crowd" are both wrong.
The core difference
Not variants of one word. An adjective and a noun that happen to sound alike.
- raucous (adjective) — harsh, loud: raucous cheering.
- ruckus (noun) — a noisy disturbance: a ruckus in the corridor.
The test
Ask whether you need a describing word or a thing. If it follows a or the and you could count it, you want ruckus. If it describes a noun, you want raucous.
Origins
Raucous is from Latin raucus, hoarse — the same root as raucity. Ruckus is American, nineteenth-century, probably a blend of ruction and rumpus. So they are not related at all; the resemblance is coincidence.
Frequently asked questions
Is "a raucous" correct?
No. Raucous is an adjective. For the noun you want ruckus.
Where does ruckus come from?
Nineteenth-century American English, probably blending ruction and rumpus.
Are raucous and ruckus related?
No. Raucous comes from Latin raucus, hoarse; ruckus is a much later American coinage. The similarity is coincidental.