When you start thinking about London place names, you realise that they pose endless questions. Was Cheapside once cheap? Who were the knights in Knightsbridge? And where does a name like Piccadilly come from?

Names like this – indeed, any place names – are inextricably linked to history. They answer questions such as "why is it here?" or "who lived here?" London place names are more fun than most, because they cover a period of over 2,000 years and tell us all sorts of things about how our capital developed.

The early history of London is vague. We know that the Romans called it Londinium and that it was a busy trading centre in their time. But Londinium is a Latin version of an older Celtic name; attempts to explain its meaning have occupied scholars for centuries. Current best guess is "settlement at the unfordable part of the river", which would certainly be geographically accurate: the lowest fordable point on the Thames was about two miles upstream from Londinium.

Also lost in the mists of time is the meaning of Thames, which may mean something as simple as "flowing". A fairly basic name for a river, you might think, but then it would have been the only major one that the early inhabitants of London knew. Modern-day Londoners still refer to it as "the river", as if it were the only one that existed or at least the only one that mattered, and this may well have been the rationale for the Celts of 2,000 and more years ago.