Lennyn is a modern American spelling of Lennon, the Irish surname Ó Leannáin, which is usually explained as "lover" or "sweetheart"; the surname became a first name in honor of John Lennon. The -yn ending turns it into a girls' name in the style of Emersyn and Kamryn. Lennyn was almost unknown before 2015 and has climbed quickly to number 1552 among US girls' names in 2025, its highest rank so far.