1934, from U + turn (n.). So called in reference to the shape of the path described.
Entries linking to u-turn
U
for 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).
The substitution of Middle English -o- for Old English -u- before -m-, -n-, or -r- was a French scribal habit before minims to avoid misreading the letters in the old style handwriting, which jammed them together. The practice transformed some, come, monk, tongue, worm.
turn n.
c. 1200, "action of rotating," from Anglo-French tourn (Old French torn, tour), from Latin tornus "turning lathe;" also partly from turn (v.). Meaning "an act of turning, a single revolution or part of a revolution" is attested from late 15c. Sense of "place of bending" (in a road, river, etc.) is recorded from early 15c. Meaning "beginning of a period of time" is attested from 1853 (as in turn-of-the-century, from 1921 as an adjectival phrase).
Sense of "act of good will" is recorded from c. 1300. Meaning "spell of work" is from late 14c.; that of "an individual's time for action, when these go around in succession" is recorded from late 14c. The automatic automobile turn-signal is from 1915. Turn-sick "dizzy," is attested from early 15c. Phrase done to a turn (1780) suggests meat roasted on a spit. The turn of the screw (1796) is the additional twist to tighten its hold, sometimes with reference to torture by thumbscrews.