lie-in
"Having a lie-in" means "sleeping in", sleeping late.

• "Parents having a lie-in, I suppose..." (GF7)

lift
U.S.: An elevator. Ironically, the meanings of 'lift' and 'elevator' are reversed between the U.S. and the U.K.; 'elevator' in the U.K. refers to what in the U.S. would be called a 'lift' put into a shoe.

• With a great jangling and clattering a lift descended in front of them... (OP7)

loo
U.S.: restroom or toilet.

• "She does it at mealtimes, in the loos, all over the place."
  - Hermione, speaking of Cho (OP21)

lot
Used to refer to people in groups, as in, "you lot", "our lot".

• "I'm not s'pposed ter talk abou' it, no, not even ter you lot."
  - Hagrid (GF37)

• "He's off with some of your lot, isn't he?"
  ...
  "Off with some of our lot?" (DH3)

lurgy
Usually referred to in the phrase "the dreaded lurgy" but which has come to mean any unspecified minor illness. The term comes from a (non-existent) highly infectious disease frequently referred to in the British 1950s - 1960s radio comedy series the Goon Show (NSOED).

• ...the possibility that Zacharias Smith, who had so far failed to maintain possession of the Quaffle for longer than a minute, was suffering from something called "Loser's Lurgy"
  (HBP19)