A child's name that never left adult English
Daddy remains, for many speakers, the first word they used for a father: informal, affectionate, and slightly childish even when adults keep it. In current English it can also name an older male partner, a financial protector, or, in slang, the most impressive figure in a group. Those later senses sit on a simpler fact. The origin of the word daddy is almost entirely home-grown. English did not import a ready-made Latin or French pet form. It built one from dad. Father stayed the word of documents and ceremony. Daddy stayed the word of the room.
Dad plus a familiar ending
Standard dictionaries treat daddy as a derivation inside English: the noun dad with the hypocoristic suffix -y. That is the same ending that turns mom into mommy and dog into doggy. It comes down through Middle English from Old English -ig. It is not a Latin-style grammatical diminutive so much as a mark of intimacy. It makes a name smaller, closer, and easier to say in the mouth of a child.
Written evidence for both dad and daddy clusters around 1500. Dates given for daddy often sit in the 1490s or at the turn of the sixteenth century. Oxford sources describe the noun as formed from dad and the -y suffix. A wrinkle in the record is that some of the earliest printed forms of the longer word appear as early as the short stem, or even earlier. That does not mean the pet form was coined first in speech. Nursery words reach print slowly. A fond name can be written down while the bare stem is still mostly spoken.
By the early 1520s John Skelton was already using a spelling along the lines of dady for a father in a comic, domestic register. From then on the word is easy to find in informal verse, later in letters and fiction: a child's address, a working-class father, a man spoken of with fondness rather than ceremony.
Baby talk, not a clipped form of father
Dad is the harder object. Historical dictionaries call it probably imitative or expressive: a child's early syllables, the same da-da that infants produce whether or not a language already has a word for father. Printed dad is about as old as printed daddy, around 1500, but the type is likely much older. Similar shapes turn up in many languages that are not copying English.
The parallels are real, and they are not all related in the usual family-tree way. Welsh tad, which becomes dad under soft mutation, including in vocative address, and Irish daid sit nearby in the British Isles. Latin tata, Greek tata as a youth's address to an elder, Sanskrit tata, Lithuanian tete, and Czech tata look like the same nursery pattern. Germanic languages have their own cluster from Proto-Germanic atto: Middle High German tate, Bavarian tatte, North Frisian forms such as aatj and taatje. Gothic used atta for father, another baby-word that had already displaced a more formal inherited term.
Whether English dad is a Celtic loan, a continuation of that Germanic father-word (sometimes proposed via an unattested Old English atta with later reshaping), or an independent child-speech invention is not settled. The Celtic account is attractive because Welsh tad and dad are close in form and geography. The baby-talk account is attractive because the same syllables keep appearing in unrelated languages. The Germanic account is more speculative for English, because the Old English form is not actually on record. Responsible histories leave the three on the table rather than picking a winner.
What can be said with more confidence is what daddy is not. It is not a worn-down piece of father. Father is the inherited Indo-European word: Old English faeder, from Proto-Germanic fader, from a PIE root that also gives Latin pater. Daddy grew up beside that formal word, as mama and mommy grew up beside mother. English kept both layers.
From the nursery into slang
Once daddy was established as an intimate father-word, English did what it often does with family nicknames: it stretched them onto men who were not fathers.
By the early eighteenth century the word could address an older man in a teasing or slightly patronizing way. In the nineteenth century it already had a figurative use for the biggest or finest specimen, still alive in phrases such as the daddy of them all. Mid-nineteenth-century theatre slang called a stage manager daddy. Prison and reformatory English later used it for the inmate who dominated a wing by force.
American slang added a sexual and economic overlay. In early-twentieth-century African American usage, daddy could mean a husband, a lover, or a sexually attractive man, without always implying great age. Jazz English produced daddy-o by 1925, in Eugene O'Neill, and spread it more widely in the late 1940s through lyrics. Sugar daddy is a 1920s American coinage. Sugar was already slang for money, and newspapers in 1923, during coverage of the Dorothy King murder in New York, brought heavy sugar daddy and then sugar daddy into print. Oxford's earliest evidence for the compound sits in that decade. Stories that pin the phrase on a particular heiress or on a candy bar are later coloring. The documented path is Broadway and tabloid slang, not a brand.
Later English recycled the same intimacy for a dominant male partner, often older, addressed as daddy. Those senses extend the protector-provider reading; they are not a separate history.
Even daddy-long-legs is a metaphor from the same stock. In Britain from 1814 it named the crane-fly; in the United States by 1865 it named the harvestman. The image is the spindly adult, not the parent.
A Gothic nickname with the same impulse
The most telling parallel is not a modern slang sense but an older Germanic one. The name Attila, by which the fifth-century Hunnic ruler is known in European languages, is Gothic: atta, father, plus a diminutive ending. It meant something close to little father, a tribal pet name with the same nursery syllable English later dressed in -y. English did not borrow Attila's nickname. It independently took a child's father-sound, attached its own familiar suffix, and then let adult slang keep borrowing the child's word whenever it needed a man who provides, protects, or simply looms largest in the room.