rabbiting
"To rabbit" means to talk incessantly or gossip idly. It's commonly used among Cockney folk, and is an example of rhyming slang where the actual rhyming word is being omitted; the full phrase is "rabbit and pork".

• "I'm not promising I'll tell yeh anythin', mind, but don't go rabbitin' about it here, students aren't s'pposed ter know." (PS14)

rally
U.S.: convention.

• "It's like some sort of...I dunno... like some sort of rally."
  - Mr Roberts on the World Cup (GF7)

register
An official record book.

• Moody took out a register, shook his long mane of grizzled grey hair out of his twisted and scarred face and began to call out names (GF14)

revision
Reviewing work already learned or done, especially to prepare for an examination (NSOED).

• They then spent over an hour revising Summoning Charms, which according to Professor Flitwick were bound to come up in their O.W.L. (OP13

• while teachers were firing revision questions at the class (OP28

• Hermione, as was her custom, had spent a large part of the day drawing up revision timetables for the three of them (OP29

rich
Much or too much.

• "You know we're not supposed to do spells outside school - "
  "Bit rich coming from you," said Harry, staring at the floating car.
  (CS3) (British edition only)

rock cake
A kind of fruitcake. It's supposed to look something like a rock, and even to have a hard surface, but not to resemble a rock quite as closely as Hagrid's version seems to.

• The rock cakes almost broke their teeth (PS8)

row (noun or verb)
As a noun, can refer to any loud noise or commotion, but when referring to something people do means a very heated quarrel; the verb sense means "quarrel, argue". (NSOED). It's worth mentioning that "row" used in these senses rhymes with "cow", not with "low".

• "Percy and Dad had a row" (OP4)

rubbish
Trash, garbage. Also used figuratively to mean "nonsense".

• "Quidditch - what is this rubbish?"
  - Vernon reading a letter from Molly Weasley (GF3)

rucksack
U.S.: backpack.

• Mrs. Weasley was still glowering as she kissed Mr. Weasley on the cheek, though not nearly as much as the twins, who had each hoisted their rucksacks onto their backs and walked out without a word to her (GF6)

• taken a small wooden wireless out of his rucksack and started to try and tune it (DH20)

ruddy
A milder, dialectal variation on the swear word, "bloody," ruddy referring to something being red-coloured (like blood). Similar and slightly more polite version of "bloody" or "damned," (from blood, Old English = rudig).

• "Ruddy stargazers. Not interested in anythin' closer'n the moon." (PS15)

• "Got to have that ruddy tail removed before he goes to Smeltings"
  - Vernon (PS6)

• "THAT RUDDY OWL!"
  - Vernon (CS3)

ruff
A disc-like, starched frilly collar, popular in the 16th century (such as worn by Elizabeth I and Shakespeare in typical portraits).

• "A ghost wearing a ruff and tights had suddenly noticed the first-years." (PS6)

runner, do a
To run away.

• "That's why he was gone when Harry and Dumbledore got there. He'd done a runner."
  - Ron on Mr. Crouch (GF29)

runner bean
U.S.: string bean.

• Mrs. Next-Door's runner beans (PA2)