Un.

Ufor historical evolution, see V. Used punningly for you by 1588 ["Love\\'s Labour\\'s Lost," V.i.60], not long after the pronunciation shift that made the vowel a homonym of the pronoun. As a simple shorthand (without intentional word-play), it is recorded from 1862. Common in business abbreviations since 1923 (such as U-Haul, attested from 1951).