An emphatic expression meaning roughly 'isn't this enough?' or 'as if to say this much' — used to stress an extreme amount or degree (often followed by phrases like '...to the point that...').
彼はこれでもかというくらいプレゼントを用意して、彼女を驚かせた。
He prepared gifts practically to the point of 'isn't this enough?' and surprised her.
料理にこれでもかと塩を入れたら、食べられなくなった。
I ended up putting salt in the dish like 'isn't this enough?' and it became inedible.
彼女はこれでもかというほど笑顔を振りまいて、場の雰囲気を和ませた。
She spread smiles as if to say 'look at this,' easing the atmosphere.
広告では商品をこれでもかと褒めちぎる表現がよく使われる。
Ads often use praise piled on to the point of 'isn't this enough?'