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.
"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.
Standard humorous spelling using katakana for イカ and kanji for 金玉.
Kanji spelling for イカ; less common but still seen.
A jocular expansion of そうはいかん, “that will not happen,” using the similar-sounding イカの金玉, “a squid’s testicles,” as a rhyme.