めったに～ない means 'rarely' or 'hardly ever'. It emphasizes that an action or state occurs very infrequently. The adverb めったに itself is not negative, but it must be followed by a negative predicate. This pattern is used in both speaking and writing, but it is more common in everyday conversation to express personal habits or experiences.

You can use it with verbs, adjectives, and nouns. For verbs, change to the negative form (e.g., 行かない, 食べない). For い-adjectives, use the negative form (e.g., 忙しくない). For な-adjectives and nouns, use ではない (e.g., 静かではない, 学生ではない). The nuance is that the frequency is extremely low, often less than 'sometimes' but more than 'never'.

Note that めったに is often used with a positive expectation, so it sounds natural to say 'I hardly ever eat meat', but not 'I hardly ever eat breakfast' if you actually skip breakfast every day—you would use 'never' instead. The pattern implies that the action does occur, but very rarely.