Denni is a variant of Denny or Denise, from Dionysios, meaning devoted to Dionysus.
Denni is a softened, affectionate variant of Dennis or Denise, a name whose roots extend all the way back to Dionysus — the ancient Greek god of wine, ecstasy, theater, and transformation. Dionysius, the Latinized form, was adopted by early Christians partly because of Saint Denis (Dionysius), the third-century bishop of Paris who became France's patron saint and whose martyrdom at Montmartre is one of the founding legends of French Catholicism. Through Saint Denis, the name spread across medieval Europe with remarkable speed and depth.
In English-speaking countries, Dennis enjoyed its peak popularity in the mid-20th century, carried partly by the cheerful cultural weight of Dennis the Menace — the comic strip launched in 1951 whose mischievous blond protagonist gave the name a distinctly boyish, irrepressible quality. The feminine Denise flourished in the same postwar decades, particularly in France and the United States. Denni collapses both genders into a single gentle form, the doubled final 'i' giving it a playful, affectionate tone.
As a standalone name rather than a nickname, Denni has the quality of many single-name variants: it feels personal and lived-in, like a name that was earned rather than assigned. It suits people who carry warmth and approachability as their natural register, and its relative rarity today makes it feel fresh despite its ancient origins. From a god of revelry to a medieval martyr to a comic strip kid — the name has traveled a wonderfully strange road.