5 5. The meaning of Kailen is  Slender . Its origin is  Variant of the Modern English name Kaylyn . Another meaning of Kailen is  From the surname McAlan . Its origin is  Variant of the Modern English name Kallen .This name is mostly being used as a unisex name. Last year it ranked 5,638th in the U.S. Social Security Administration list of most popular baby names. 5. The meaning of Kailen is  Slender . Its origin is  Variant of the Modern English name Kaylyn . Another meaning of Kailen is  From the surname McAlan . Its origin is  Variant of the Modern English name Kallen .