expression
you wish!; that's not going to happen; not a chance
A jocular, punning phrase used to dismiss someone's hopes or plans. It plays on the similarity between そうは行かぬ (that won't do) and そうはイカの金玉 (that's a squid's testicles), creating a humorous, slightly vulgar rejection.
「今度こそ宝くじが当たるよ！」「そうはイカの金玉だね。」
"I'm going to win the lottery this time!" "Yeah, you wish!"
彼が謝るなんて、そうはイカの金玉だよ。
Him apologizing? Not a chance.
The original phrase meaning 'that won't do' or 'it won't go that way'; そうはイカの金玉 is a pun on this, replacing 行かぬ with イカの金玉 for humorous effect.
A pun on そうは行かぬ (sō wa ikanu, 'that won't do'), replacing 行かぬ (ikanu) with イカの金玉 (ika no kintama, 'squid's testicles') for humorous effect. The phrase is jocular and not derived from any historical or literal meaning.