Variant of Jocelyn, from a Germanic tribal name (the Gauts) adopted into Norman French.
Joslyn is a variant of Jocelyn, a name with a rich and somewhat tangled etymology. It descends from the Old French Joscelin, which was introduced to England by the Normans and ultimately traced to a Germanic tribal name, the Gautzelin or Gauzelin, referring to a member of the Gaut people — one of the Gothic tribal groups whose migrations shaped post-Roman Europe. The name was used for both men and women in medieval England, as was common with many names of that era before gender distinctions in naming became more rigidly enforced.
Joscelin de Courtenay, a twelfth-century Crusader prince who ruled the County of Edessa, is among the name's more dramatic historical bearers. The feminization of Jocelyn in English-speaking countries gathered momentum through the nineteenth and twentieth centuries, and it enjoyed particular popularity in Britain and North America from the 1950s through the 1980s. The spelling variants proliferated as the name traveled: Jocelyn, Joselyn, Josslyn, and Joslyn each represent slightly different aesthetic preferences, with Joslyn tending toward a more streamlined, modern sensibility.
The -lyn ending places it in a large family of names — Carolyn, Marilyn, Evelyn — that carry a specifically mid-century American femininity while also feeling current enough for contemporary use. S. Surgeon General under President Clinton, who gave the name associations with intellectual boldness and public service.
In fiction and popular culture, Jocelyn and its variants have appeared widely enough to feel familiar without being ubiquitous — a useful quality in a given name. Joslyn in particular, with its dropped 'c,' feels brisk and contemporary, appealing to parents who want a name with genuine medieval depth but a clean, forward-facing profile.