Diminutive of Robert, from Germanic 'hrod' (fame) and 'berht' (bright), meaning bright fame.
Robby is the warmest diminutive of a name that has been practically everywhere. Robert descends from the Old High German *Hrodebert*, a compound of *hrod* (fame, glory) and *beraht* (bright) — so Robby, at its etymological core, is a term of affection applied to someone marked for bright renown. The Normans brought Robert to England after 1066, where it became one of the most common names in the medieval lexicon, giving rise to the surnames Robinson, Roberts, Robertson, and the word 'robot,' via Czech, as a kind of dark linguistic joke about labor.
The -y and -ie diminutives proliferated in the twentieth century, carried by figures like Robby the Robot from *Forbidden Planet* (1956), whose name became a pop culture touchstone for gleaming, good-natured futurism. Robby Krieger of the Doors gave the spelling a rock credibility, while Robbie Williams made the softer spelling a stadium-filling presence. The name also appears in countless coming-of-age narratives as the name of the kid next door — accessible, unpretentious, immediately likeable.
As a full given name rather than a nickname, Robby has a different character from Robert: more immediate, more playful, less formal. Parents who register it on a birth certificate are often signaling that they want the warmth without the stiffness, a name for someone who will be comfortable in sneakers and boardrooms alike. It is a name that has never been fashionable in a self-conscious way, which is precisely what makes it endure.