Verb
knock oneself out (third-person singular simple present knocks oneself out, present participle knocking oneself out, simple past and past participle knocked oneself out)
(idiomatic, informal) To go ahead; to do as one pleases.
"These cookies are my favorite! Can I have some?" "Sure, knock yourself out." (i.e., have as many as you want)
(sarcastically, indicating lack of interest) "I'm going to the store." / "Knock yourself out."