I found this written by Michael Skeet in Oct 2004 and it was posted on The Aerodrome on the same date.
"Fug" boots are generally considered to have been invented by Lanoe Hawker (who, since he was flying a DH-2 with no warm engine between him and the cold, probably had a lot of incentive working for him). They were basically a kind of hip-wader, but lined with fur or fleece (and sometimes covered that way as well). Even after the introduction of the Sidcot flying suit, pilots continued to wear them for added protection against the cold, especially in the non-summer months.
"Fug," by the way, is a slang term meaning close and fusty, as in the warm stuffiness of an RFC mess (in which, we are told, the windows were never opened, even in summer, because pilots treasured being warm).