talk through one's hat

Meanings

1. To speak lacking expertise, authority, or knowledge; to invent or fabricate facts.
2. To assert something as true or valid; to bluff.

Etymology

Attested from the late 19th century in the United States in the sense of “bluff”; the sense “speak without authority or knowledge” developed later. Although some people speculate a connection to a former requirement that British Members of Parliament wear hats, the connection is implausible.