quack

Concise Oxford English Dictionary © 2008 Oxford University Press:
quack1

▶noun the characteristic harsh sound made by a duck. ▶verb make this sound.

– origin C16: imitative.

Concise Oxford English Dictionary © 2008 Oxford University Press:
quack2

▶noun

• 1 an unqualified person who dishonestly claims to have medical knowledge.

• 2 Brit. informal a doctor.

– derivatives
quackery noun,
quackish adjective.

– origin C17: abbrev. of earlier quacksalver, from Du., prob. from obs. quacken ‘prattle’ + salf, zalf (see salve1).