Top Definition
1. When one of McDonald's foods, drinks, products etc. somehow invades the public in a way that says, "I'm lovin' it".

2. When you feel sick after eating something from McDonald.
1. I'm walking down the road one day and I see a Ronaldmobile drive know a Mctack is coming.

2. After eating two Big macs, drinking two cokes, and 3 medium orders of fries, I think a Mctack is going to happen.
by Riunzo November 29, 2011
tacky. doesn't look good.
That sweater is so mctacks.!
by KristenDanielle.! April 16, 2009

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