It used to be a common nickname. Don't see it too often nowadays, but it's still found in rural areas, like depicted in this show. https://en.wikipedia.org/wiki/HAP#As_a_nickname
Don't mess with me, man! I know karate, judo, ju-jitsu..... and several other Japanese words.
It looks like Hap is his actual, given name. At the start of ep. 3, they show a close-up of his induction notice from the Army, ordering him to report for military service. It is addressed to Hap Collins. So I guess we can assume that Hap is his legal, given name, not a nickname.
Don't mess with me, man! I know karate, judo, ju-jitsu..... and several other Japanese words.
Happy?
Haphazard?
Hapalong?
Happenstance?
Happening?
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You have meddled with the primal forces of nature, and YOU ... WILL ... ATONE!
Long ago, I knew someone named Happy that everyone called Hap.