(adjective) Awesome, kickass, or otherwise positive. Can be used to refer to anything but is usually referring to a particular event or action. The most common usages are "epic win" or "epic failure," and some prefer to type it in all caps. Occasionally people use the phrase "Epic ___" as a stand-alone sentence or phrase, always following a story about something considered Epic.
Example 1: "He's a blackbelt in karate, so when this asshole came up to him, he totally kicked the dude's ass. You should have been there, it was fucking Epic."
Example 2: "I'm a blackbelt in karate, so this asshole came up to me and I kicked his ass. Epic fucking win."
Example 3: "That asshole from down the street got his ass kicked today."