"Quack" is a word in LAW AND LEGAL, ENGLISH

A pretender to medical skill which he does not possess; one who practices as a physician or surgeon without adequate preparation or due qualification. See El-mergreen v. Horn, 115 wis. 385, 91 N. W. 973

Hence, one who boastfully pretends to skill or knowledge of
any kind not possessed; a charlatan.

Pertaining to or characterized by, boasting and pretension;
used by quacks; pretending to cure diseases; as, a quack medicine; a
quack doctor.

The cry of the duck, or a sound in imitation of it; a
hoarse, quacking noise.

To make vain and loud pretensions; to boast.

To act the part of a quack, or pretender.

A boastful pretender to medical skill; an empiric; an
ignorant practitioner.

To utter a sound like the cry of a duck.