'apikoros' originates from Greek, specifically the name 'Epikouros' (Epicurus), which was adopted into Jewish usage (via Aramaic/Hebrew and Yiddish) to label followers or ideas associated with Epicurus.

'apikoros' changed from Greek 'Epikouros' (the name of the philosopher Epicurus) into rabbinic Hebrew/Aramaic and later Yiddish as 'apikoros'/'apikores', and was borrowed into English with the meaning preserved as a term for a heretic.

Initially it referred to a follower of Epicurus or to Epicurean philosophy; over time, in Jewish and later English usage it came to mean 'heretic' or 'one who denies core religious doctrines.'