Plain English Meaning
A metal cover around the engine's exhaust pipes that quiets the noise and warms up outside air for cabin and carburetor heating.
Full Definition
A component of the aircraft exhaust system, typically a metal shroud or chamber surrounding the exhaust pipes, that reduces engine exhaust noise and, in many light aircraft, also serves as the heat source for cabin heat and carburetor heat by transferring heat from the hot exhaust gases to outside air passing through or around it.
Context Anchor
Seen in aircraft engine, exhaust system, preflight inspection, and cabin heat discussions.
Derivation
From the verb 'muffle,' meaning to wrap something up to deaden sound. The muffler does the same job an automotive muffler does — wraps the noisy exhaust path to quiet it — with the added role of heating cabin and carburetor air.