na-adjective, noun
impertinent; presumptuous; know-it-all
Describes someone who acts clever or knowledgeable in an annoying, presumptuous way; often used for a person who gives unwanted advice or opinions. Slightly old-fashioned but still understood.
彼の賢しらな態度には腹が立つ。
His know-it-all attitude is irritating.
賢しらに口を出すな。
Don't butt in with your presumptuous opinions.
生意気 is more common and colloquial for 'cheeky' or 'impudent', while 賢しら has a nuance of acting clever or knowledgeable in a presumptuous way, and is less frequently used.
差し出がましい focuses on being meddlesome or forward, whereas 賢しら emphasizes the know-it-all, clever-acting aspect.
Derived from 賢し (sakashi, 'clever') + suffix ら (expressing a state or manner). The exact historical development is uncertain, but it has been used since classical Japanese to describe presumptuous cleverness.