This is a metonymic i.e. 'the son of Til(l)' - a pet form of the personal name Matilda from the Olde German meaning 'mighty battlemaid'. William the Conqueror's wife was a name bearer which popularised the name in Medieval court circles. Till-in and Till-ot are diminutive forms i.e. 'little Till'. The 'g' is excrescent. The surname from this source is first recorded in the latter half of the 13th century. Alternate spellings have included Tyllyng (1280) and Tulling (1327). In 1691, one John Tilling married a Margaret Joy in St. James Church, Clerkenwell, London. The latter spelling remains unvaried to the present day. The first recorded spelling of the family name is shown to be that of John Tylling which was dated 1279 The Hundred Rolls of Oxfordshire during the reign of King Edward 1 The Hammer of the Scots 1272-1307 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.