A name given to someone who likes to ruin potential fun being had. McGreets can come in many varieties. Some McGreets feel the need to ruin funny moments with their unimportant comments. Others like to use sarcasm at incorrect times to, instead of coming off as funny and witty, are seen as someone with a stick up their ass. McGreets is usually a friendly way of telling someone to 'shut the fuck up'.
McGreets: "Dude, that waitress wasn't digging you at all. Besides, she has a boyfriend. Why are you trying?"
Friend: "Listen here McGreets."