Top Definition
a guy, that gets a little too happy when he sees a girl, causing an erection then later causing a stain to appear on pants.
Bob had a case of McHappy pants when he saw sally.
by Casrox your face October 10, 2006
1. usually means someone very happy with good luck and amazing social skills

2. someone who is hilarious and extremely good at making friends
my friend is such a Mchappy pants; she's always making everyone smile and we all love her to death
by Sophizzlemadizzle September 07, 2006
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