Idk = I dont know.
Idc = I dont care.
Idr = I dont remember.
Idgaf = I dont give a fuck.
Lol = laughing out loud.
Lmao = laughing my ass off.
Lmfao = laughing my fucking ass off.
Rofl = rolling on floor, laughing
Roflmao = rolling on floor, laughing my ass off.
smh = shaking my head my off
wtf = what the fuck
Wth = what the heck
Gg = good game
Gj = good job
Gl = good luck
Hf = have fun
Cya = see ya
C = see
B = be/bee
D = Dick

I = I/eye
O = oh
P = pea
R = are
T =tea
U = you
Y = why
2 = to
4 = for
6 = Sex
8 = Ate
M8 = mate

To make briefer, shorten.

Abbreviation for abbreviate is Abbr.

A shortened form of a word or phrase.

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Types of abbreviations:

There are four main kinds of abbreviations: shortenings, contractions, initialisms, and acronyms. 1 Shortenings of words usually consist of the first few letters of the full form and are usually spelled with a final period when they are still regarded as abbreviations, for example, cont. = continued, in = inch. In the cases when they form words in their own right, the period is omitted, for example, hippo = hippopotamus, limo = limousine. Such shortenings are often but not always informal. Some become the standard forms, and the full forms are then regarded as formal or technical, for example, bus = omnibus, taxi = taxicab, deli = delicatessen, zoo = zoological garden. Sometimes shortenings are altered to facilitate their pronunciation or spelling: bike = bicycle2 Contractions are abbreviated forms in which letters from the middle of the full form have been omitted, for example, Dr. = doctor, St. = saint or street. Such forms are invariably followed by a period. Another kind of contraction is the type with an apostrophe marking the omission of letters: can't = cannot, didn't = did not, you've = you have. 3 Initialisms are made up of the initial letters of words and are pronounced as separate letters: CIA (or C.I.A.), NYC, pm (or p.m.), U.S. (or US). Practice varies with regard to periods, with current usage increasingly in favor of omitting them, especially when the initialism consists entirely of capital letters. 4 Acronyms are initialisms that have become words in their own right, or similar words formed from parts of several words. They are pronounced as words rather than as a series of letters, for example, AIDS, laser, scuba, UNESCO, and do not have periods. In many cases the acronym becomes the standard term and the full form is only used in explanatory contexts.

means a word that has been abbreviated

see abbr.

a person who abbreviates every word

To shorten a sentence, for instance LOL is short for Laugh Out Loud.
Same as FYI is short for For Your Information and etc etc etc

Stuff like that are ALL abbreviations.

The fastest way to get a "def" on urbandictionary.com.

A short version of a word