English▸ noun [in singular] the characteristic harsh sound made by a duck:
I heard a quack and saw some ducks huddled together.
▸ verb [no object] (of a duck) make a characteristic harsh sound:
ducks quacked from the lake.
▪ informal talk loudly and foolishly:
he was still quacking about vinyl's alleged superiority to CDs.
– ORIGIN mid 16th century (as a verb): imitative.