Lyndsey comes from an English place name meaning 'island of linden trees' or 'Lincoln's marsh island.'
Lyndsey is a variant spelling of Lindsay or Lindsey, a name with roots in the Anglo-Saxon and Old Norse landscape of early medieval Britain. The place name Lindsey — an ancient administrative region in what is now Lincolnshire, England — is generally understood to derive from Lindes ig, meaning roughly "Lincoln's island" or "the island of the Lindis people," with the Lindis being a local tribal group whose name may itself relate to the Old English for "lake" or "pool." The name thus begins in geography and waterways before it becomes a personal name at all.
As a surname, Lindsay was adopted by a Norman family who settled in Scotland after the Conquest, and through Scottish history it became both a noble surname and eventually a given name. Sir David Lindsay of the Mount, the 16th-century Scottish poet and Lord Lyon King of Arms, was among the most politically influential literary figures of his age, and the Lindsay name spread through Scottish aristocracy and lowland culture. When the name crossed into use as a given name in the 20th century, it was initially primarily masculine in Scotland and the UK before shifting to predominantly feminine use in North America.
The Lyndsey spelling, with its y instead of i, is a specifically American customization that emerged during the name's peak popularity in the 1970s through 1990s — a period when Lynn, Linda, and Lindsey were all flowing together in the cultural naming stream. The extra y gives it a slightly softer, more romanticized appearance on paper while leaving the pronunciation identical. Today Lyndsey feels warmly vintage: associated with the era of its peak use without being entirely dated, the spelling a small act of personalization that its bearers tend to appreciate as they grow older.