Etymology
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U-boat (n.)
1916 (said to have been in use from 1913), partial translation of German U-Boot, short for Unterseeboot, literally "undersea boat."
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boat (n.)

"small open vessel (smaller than a ship) used to cross waters, propelled by oars, a sail, or (later) an engine," Old English bat, from Proto-Germanic *bait- (source also of Old Norse batr, Dutch boot, German Boot), possibly from PIE root *bheid- "to split," if the notion is of making a boat by hollowing out a tree trunk or from split planking. Or it may be an extension of the name for some part of a ship.

French bateau "boat" is from Old English or Norse. Spanish batel, Italian battello, Medieval Latin batellus likewise probably are from Germanic. Of serving vessels resembling a boat, by 1680s. The image of being in the same boat "subject to similar challenges and difficulties" is by 1580s; to rock the boat "disturb stability" is from 1914.

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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.

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boat-house (n.)
"shed for storing boats and protecting them from weather," 1722, from boat (n.) + house (n.).
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U-bahn (n.)
German or Austrian subway system, 1938 (originally in reference to Berlin), from German U-bahn, short for Untergrund-bahn, literally "underground railway."
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jolly-boat (n.)
"small boat hoisted at the stern of a vessel," 1727; the jolly is of unknown origin, probably from Danish jolle (17c.) or Dutch jol (1680s), both related to yawl; or it may be from Middle English jolywat (late 15c.) "a ship's small boat," of unknown origin.
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*mregh-u- 
Proto-Indo-European root meaning "short."

It forms all or part of: abbreviate; abbreviation; abridge; amphibrach; brace; bracelet; brachio-; brachiopod; brachiosaurus; brachy-; brassiere; breviary; brevity; brief; brumal; brume; embrace; merry; mirth; pretzel; vambrace.

It is the hypothetical source of/evidence for its existence is provided by: Greek brakhys "short;" Latin brevis "short, low, little, shallow;" Old Church Slavonic bruzeja "shallow places, shoals;" Gothic gamaurgjan "to shorten."
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long-boat (n.)
longest and strongest boat on a sailing ship, 1510s, from long (adj.) + boat (n.).
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flat-boat (n.)
also flat-boat, 1650s, from flat (adj.) + boat (n.).
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life-boat (n.)
"boat built for saving lives at sea," especially in a shipwreck, also lifeboat, 1801 (the thing itself attested by 1785), from life (n.) + boat.
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