na-adjective
Describes acting without careful thought, often with a negative connotation. Used as a na-adjective.
滅多なことを言うな。
Don't say such reckless things.
彼は滅多な行動で
He troubled those around him with his thoughtless actions.
na-adjective
Almost always used with a negative expression (e.g., 滅多に〜ない) to mean 'rarely' or 'seldom'. The adjective itself means 'ordinary', but the negative construction is the core usage.
彼は滅多に怒らない。
He rarely gets angry.
こんなチャンスは滅多にない。
An opportunity like this is rare.
prefix
Prefix used in a few set phrases like 滅多打ち (indiscriminate beating) or 滅多切り (hacking wildly). Not freely productive.
敵に滅多打ちにされた。
We were beaten indiscriminately by the enemy.
「滅多切り」は、めったやたらに切ることを表す。
「滅多切り」 means cutting wildly and indiscriminately.