of or relating to sickly, run-down, bedridden or under the weather, in the sense that one is feeling like "a piece" of shit
Also used in terms of detestable, unworthy, despicable; not deserving of respect and/or genuine camaraderie
Tom: "Hey bro, you coming out tonight?"
Mike: "Naw dude, I drank too much last night. I feel like a piece."
Tom: "Dude, did you and Erika break up?"
Mike: "Yeah man, I found out she was hooking up with some other guy."
Tom: "Wow. What a piece."