expression, adverb
to an extreme degree; excessively; as if it weren't already enough
Used adverbially to emphasize that something is done to an excessive or over-the-top degree, often with a critical or exasperated tone. The phrase literally means 'to the extent of saying "isn't this enough already?"'.
彼はこれでもかと言うほど食べた。
He ate to an excessive degree.
その映画はこれでもかと言うほど怖いシーンが続く。
That movie has an excessive number of scary scenes one after another.
過剰に is a more formal, neutral adverb meaning 'excessively', while これでもかと言うほど is a colloquial expression with a stronger emotional nuance of exasperation or criticism.
やたらに means 'recklessly' or 'indiscriminately' and focuses on lack of restraint, whereas これでもかと言うほど emphasizes the excessive degree itself.
A set phrase combining これでもか ('isn't this enough?') with the quotative と and 言うほど ('to the extent of saying'). The expression likely originated from a rhetorical question used to express that something has already exceeded a reasonable limit.