adverb, adverb taking the 'to' particle
indiscriminately; recklessly; excessively
Describes doing something without restraint, thought, or moderation: acting blindly, at random, or in a profuse, over-the-top manner. Often carries a negative or critical tone.
彼はやたらめったらにお金を使う。
He spends money recklessly.
やたらめったらに物を買わないほうがいい。
You shouldn't buy things indiscriminately.
やたら is a more common and slightly less emphatic adverb meaning 'excessively' or 'at random'. やたらめったら is a reduplicated, more intense version with a stronger sense of disorder or recklessness.
めちゃくちゃ can mean 'messy', 'absurd', or 'excessively', but it is broader and can also describe physical disorder. やたらめったら focuses on indiscriminate or reckless action.
Reduplication of やたら (excessively, at random) with the emphatic element めったら, likely a sound-symbolic extension. The exact derivation of めったら is uncertain.