あっそ

もちろん冗談（じょうだん）と思（おも）って彼（かれ）は「あっそ」と流（なが）していましたけどかなり嫌（いや）だったそうです。ってゆうか冗談（じょうだん）でもそんなこと言（い）うな！

Of course he thought it was a joke and let it slide with a "hm?" but he was seriously annoyed. I mean, just don't say things like that even as a joke!