i-adjective
Used to describe something that happens very infrequently or is hard to find. Often appears in the pattern 滅多に〜ない (rarely does ~).
彼は滅多に怒らない。
He rarely gets angry.
こんな機会は滅多にない。
An opportunity like this is rare.
稀 (まれ) is a na-adjective/noun meaning 'rare' or 'uncommon', often used in formal or written contexts. 滅多にない is more colloquial and emphasizes infrequency of occurrence.
滅多 (めった) is a word meaning 'reckless' or 'thoughtless', but in this phrase it has shifted to mean 'rarely' or 'hardly'. The exact derivation is uncertain; the combination with ない forms a common negative expression.