These adjectives mean "not constant," "not regular," "not habitual," "occurring not often": "His visits to his parents were infrequent (or unfrequent)." Infrequent is much more often used and is preferred. In another adjectival form, however, unfrequented (meaning "not regularly visited," "usually empty") is correct, and infrequented is not: "This lovely campsite is far off the trail and therefore unfrequented." "The park became Uttered with refuse and was unfrequented until it was cleaned up."
Dictionary of problem words and expressions. Harry Shaw. 1975.