U meaning in Internet Slang ?

Answer: What is You mean?

In Modern English, you is the second-person pronoun. It is grammatically plural, and was historically used only for the dative case, but in most modern dialects is used for all cases and numbers.

reference

Full Form	Category
Uncertain	Academic & Science
Uracil	Academic & Science
Unified atomic mass unit	Academic & Science
Unexcelled	Academic & Science
Unsatisfactory	Academic & Science
Undergraduate	Academic & Science
University	Academic & Science
Undecided	Academic & Science
Undergirding	Academic & Science
Uranium Participation Corporation	Business
U S Airways Group, Inc.	Business
Unlimited	Business
Undetectable	Chemistry
Uranium	Chemistry
Universal Pictures	Community
Use	Community
Uhf	Community
Union	Community
Unity	Community
Unification	Community
Urban	Community
Understanding	Community
Universal	Community
Unconditional	Computer Assembly Language
Up	Computer Assembly Language
Unwind Tape	Computer Hardware
Userid	Computing
Unlocked	Computing
Undefined	Computing
Unicode	Computing
Users	Computing
Integrated Circuit (circuit Diagram Marking)	Electronics
Subsampled Raw Yuv File Bitmap Graphics	File Type
Unhappy	General
Upbuilding	General
Ursula	General
The Uncompensated	General
The Usual	General
Unplasticised	General
Underwear	General
Uncommon	General
Unlicensed	Governmental
Uncompromising	Governmental
Unavailable	Governmental
Unsigned	Governmental
Uncommitted	Governmental
Uninduced	Governmental
Unredacted	Governmental
Üniversite	International
Umleitung	International
U-Bahn	International
Und	International
You	Internet Slang
Useless	Jobs
Upsilon	Maths
Units	Measurement Unit
Unusual	Medical
Ultralente insulin	Medical
Under	Medical
Undetected	Medical
Wind Speed	Military and Defence
Unterseeboot	Military and Defence
Unknown	Military and Defence
Unreliable	Military and Defence
Uphill	Military and Defence
Unclassified	Military and Defence
Unmodified	Military and Defence
Unidentified	Military and Defence
Uninterpretable	Physics Related
Uninstall	Softwares
Underfloor	Space Science
Undefeated	Sports
Unassisted put out	Sports
United	Sports
Ultimate	Sports
Unofficial	Sports
Uniform	Transport & Travel