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
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.