1.
rarely, seldom, hardly ever
Used with a negative verb to mean 'rarely' or 'hardly ever.' Always appears in the pattern めったに〜ない. This is by far the most common usage.
2.
reckless, indiscriminate, thoughtless
Acting without careful thought or consideration. Used as a na-adjective to describe careless or random actions. Less common than sense 1.
Usage
Sense 1 (滅多に〜ない) is extremely common and one of the standard ways to say "rarely" in Japanese. It always requires a negative ending.
Sense 2 is less common in modern Japanese but appears in set expressions like 滅多打ち (indiscriminate beating) and 滅多なことを言う (to say something reckless).
The kanji 滅多 is often written in hiragana as めった.