The name Kaelin is believed to have originated from the Scottish Gaelic and Irish Gaelic languages. It is thought to be derived from the Old Irish word 'caol' or 'caelan', which means 'slender' or 'thin'.

The name Kaelin has been in use since at least the 16th century in Scotland and Ireland. It was originally a masculine name but has become more commonly used as a feminine name in modern times.

One of the earliest recorded instances of the name Kaelin is found in the Annals of Ulster, an ancient Irish chronicle that covers events from the 5th to the 16th century. In this text, the name is spelled 'Caelán'.

In the 17th century, the name Kaelin appears in the Scottish Gaelic poetry of Alasdair MacMhaighstir Alasdair, a renowned Scottish poet and political activist.

One of the earliest known individuals to bear the name Kaelin was Caelán of Leinster, an Irish poet who lived in the 8th century. He is best known for his work 'The Poems of Caelán', which is considered a significant contribution to early Irish literature.

Another notable figure with the name Kaelin was Caelán Bán mac Buachaill, an Irish poet and historian who lived in the 15th century. He was a member of a prominent family of poets and is known for his work 'The Book of Caelán Bán'.

In the 19th century, Kaelin was the name of a character in the novel 'The Fair God' by Lew Wallace, published in 1873. The character was a Native American princess, which helped to popularize the name in the United States.

Other historical figures with the name Kaelin include Kaelin Deery (1892-1965), an Irish politician and member of the Irish Republican Army, and Kaelin Guédry (1912-1994), a French-Canadian soldier who served in World War II.

While the name Kaelin has been in use for centuries, it has become more widely known and popularized in recent times, particularly in the United States and other English-speaking countries.