Despite moving to Paisley in January, Magennis joined up with his old Pittodrie mates for last Sunday's League Cup final and was spotted sprinting across the Parkhead pitch to join his jubilant former team-mates as they partied after beating Inverness on penalties.
But his new Saints colleagues have been quick to dub the Northern Irishman 'John Terry' in reference to the Chelsea captain's antics when the Blues won the Champions League in 2012.
The former England skipper stripped off his tracksuit to reveal a full playing kit, even though he was suspended for the Munich final.
McGinn laughed as he claimed the ribbing Magennis has suffered at their Ralston training ground this week will ensure he thinks twice about joining in with the Dons' celebrations if Derek McInnes' men also go on to win the William Hill Scottish Cup later this season.
He said: "Josh has taken a lot of stick this week, and rightly so. I couldn't believe my eyes when I saw him doing a 100-metre sprint over the pitch to hug his ex-manager.
"He played 10 minutes in the competition, which doesn't merit a mad sprint across the pitch.
"The fans haven't been giving him as much stick as the players have. He's been absolutely terrorised this week and rightly so. I doubt he'll be doing the same again if Aberdeen get to the Scottish Cup final."
While there have been plenty of laughs at Magennis' expense this week, McGinn knows St Mirren's current league position is no laughing matter.
Danny Lennon's men threw away a 2-0 lead with 12 minutes remaining against Dundee United last week to drop into the relegation play-off spot and now face Celtic on Saturday.
But Scotland Under-21 cap McGinn admits Saints are not used to finding themselves in winning positions.
"Losing to United was a blow because we had worked so hard to get into a winning position," he said.
"It's not like us to be 2-0 up and maybe it took us by surprise. We were cruising and lost a goal that should not have stood. But we shouldn't have fallen apart the way we did.
"We know what we are capable of but we haven't shown that enough this season. When we press teams the way we did against United in the first 78 minutes, it shows what position we can get ourselves into. But after that, we showed our negative side, losing three goals in such a short space of time.
"We have got that reputation of being a 'maybe' team. That's frustrating. We need to try to make sure we are known as a good side and not just a nearly-good side.
"We need to give it a go against Celtic. They might turn up and think it will be another easy three points. But we know we have the players capable of shocking Celtic into making mistakes.
"Every game now is a cup final for us. Partick and Ross County have been picking up wins and we need to do the same. But we know we are better than finishing second bottom."
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