It's a bit of a reach tbh. "jopa" (жопа) would be a crude way to say "ass". Then some schoolyard nicknames would sometimes have a suffix -lin or -in (-лин,-ин) as general creative addition to create nickname. Unless I'm clueless about of some slang usage, or grandparent refered to some other Slavic language (not russian), "Joplin" (жоплин) doesn't mean anything directly.

Czech here and no idea what it should be. Not even close to some profanity.