This very interesting Welsh late medieval surname is a curious dialectual formation of the Celtic 'Gwyn' - meaning 'the fair haired one' and the Saxon (and later Norman) 7th century William, itself originally a compound which translates as 'mind-helmet'. The name development has included John Ap-Gwilym, the Chancellor of St. Davids (1351), Lewis Gwilliam (1631 Shropshire) and 1763 Robert Gwillim of East Cheshire. The first recording shows the complexity of Welsh names, it breaks down as Gwyn-William the son (ap) of John's (jan) son (kin). The plurality may also be an additional patronymic son of i.e. Williams - the son of William. The first recorded spelling of the family name is shown to be that of Jankyn ap Gwilliam which was dated 1391, in G.P.Jones, "The Extent of Chirkland" 1391 - 1393 during the reign of King Richard 11 Richard of Bordeaux 1378-1400 Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.