fairy lights - fanciable - fancy - Father Christmas - fen - fifth-year - first-year - flap - flat (noun) - flutter (adj) - flutter (verb) - football - fourth-year - fringe - frock-coat - front three - fug
Depends on context. As a verb, this can mean 'want' or 'like' (usually when referring to things or actions), but when referring to people, indicates romantic attachment (think 'footloose and fancy-free'). As a noun, a 'fancy' is a kind of small cake.
The Fenland is an area of low-lying, very flat land (some of it below sea-level) in eastern England. Although nearly all of it is now drained and is a very fertile agricultural area, historically it was a place of marshes extending as far as the eye could see and well beyond. The counties included in the Fenland are: Cambridgeshire (almost all of it), Huntingdonshire (all of it), Norfolk (parts in the western area), Lincolnshire (south); and Bedfordshire (parts of the northern area). Historically, Oliver Cromwell was known to the people of Cambridgeshire as "the Lord of the Fens" for his efforts on their behalf early in his career in Parliament. Famous works of fiction set in the Fenland include Dorothy L. Sayers' The Nine Tailors.
Drainage has resulted in shrinking of the soil, to the extent that some rivers, notably much of the Norfolk part of the Great Ouse, flow well above the surrounding land and are contained only by massive earthen embankments. The Isle of Ely, miles from the sea, is so-named because it was an island in the surrounding miles of fens.
Fifth-years were allowed to be out in the corridors until nine o'clock (OP18)
"I'm all of a flutter" (PS5)
Bet - "Fancy a flutter" means "Would you care to make a bet"
Once, Aunt Petunia, tired of Harry coming back from the barber's looking as though he hadn't been at all, had taken a pair of kitchen scissors and cut his hair so short he was almost bald except for his fringe, which she left 'to hide that horrible scar' (PS2)
...he stared at his feet to avoid catching anyone's eye, flattening his fringe as he did so (OP7)
a sallow-faced wizard with a short black fringe (OP22)
A man's double-breasted long-skirted coat not cut away in front; now worn chiefly on formal occasions (NSOED).
...he was also wearing the strange assortment of clothes so often chosen by inexperienced wizards trying to look like Muggles: in this case, a frock coat and spats over a striped one-piece bathing-costume (HBP10)
In rugby, the forwards who make up the first row in a collision with their opponents are referred to as the front row. By extension, "front three" in this context means the three players who will be first to meet the opposing team.
A thick, stuffy smelly atmosphere (NSOED). An example would be 221 B Baker Street while Sherlock Holmes was thinking about a three-pipe problem - he was given to smoking while pondering such matters.