Recorded in a wide range of spellings including Gillean, Gillian, Gilleon, Guilaem, Guillain, Guilayn, Guilyan, and Guillem, this is a Gaelic or Celtic surname, with two possible origins, both personal names. If Gaelic the derivation is believed to be from the pre 10th century diminutive word "giollain", from "giolla" meaning literally boy or lad. This was either a term of endearment which became a personal name, or it may have been a status name for a companion or servant. In medieval times the name would seem to have been prefixed with either Mac meaning son of, or O', the descendant of, to give a meaning equivalent to the son of the son (or descendant) of the Lad. Most Gaelic surnames originate from a name, often a nickname, for the first chief of the clan or sept. If Celtic or Welsh, the derivation is believed to be from the Norman surname William, introduced into Britain after the Invasion of 1066, and into Wales from the 12th century. This is through the medieval dialectal development of Gwilliam, Gwilym or Gwylym. Early examples of the surname recordings from both these origins include: Rice ap Guillan of Radnorshire, Wales, in 1533, John Guilyan who married Elspe Riske at Govern, Lanarkshire, Scotland, on December 30th 1695, and John Guilaem of Belfast, Ulster, Northern Ireland, on December 14th 1864. Over the centuries spelling at its best has been erratic, and local dialects very thick. This has lead to the development of variant or "sounds like" spellings, some far removed from the original forms.

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