Cadie is likely a diminutive or variant of Katie, from Katherine, traditionally interpreted as pure.
Cadie is a diminutive form with several possible roots, the most prominent being a shortened and softened version of Cadence, Cady, or the Irish and Scottish Gaelic names Caitríona and Catrìona — themselves forms of Catherine, which traces back through Latin Catharina to the Greek Aikaterine. The etymology of Catherine is famously debated: one tradition links it to Hecate, the Greek goddess of magic and crossroads; another derives it from the Greek katharos, meaning "pure." The "-ie" ending is a deeply familiar diminutive in British and Irish naming traditions, suggesting affection and intimacy.
Cady in its own right became famous in the nineteenth-century United States through Elizabeth Cady Stanton (1815–1902), one of the founders of the women's suffrage movement, which gave the name a distinctly American feminist resonance for generations of parents. The softer spelling Cadie strips away some of that historical gravity while keeping the sound warm and friendly. In Scottish usage, "Cadie" or "Cady" was also a term for a messenger boy or errand runner, a word that entered English from French cadet.
Today Cadie occupies the charming territory of the unexpected nickname-name — a name that sounds like it belongs to someone you'd want to know, breezy and approachable but not without depth. It suits a wide range of personalities and ages, and its rarity in formal birth records makes it feel like a considered, individual choice rather than a trend. Parents who pick it often appreciate its lightness and its whisper of old-world diminutive tradition.