jemandem einen Bären aufbinden
to tell someone a tall tale; to pull someone's leg
Origin
Origin not clear; probably from the Germanic root "bar-" (= burden, to carry), folk-etymologically reinterpreted as "Bär" (bear). Add to this folk anecdotes and the 19th-c. Viennese "Bären" (tall tales).
Examples
Glaub ihm nicht, er will dir nur einen Bären aufbinden.
Don't believe him, he's just pulling your leg.