U-boat (yo?o?b?t?)n. A submarine of the German navy.[Translation of German U-Boot, short for Unterseeboot : unter, under (from Middle High German under, from Old High German untar; see n?dher- in Indo-European roots) + See, sea (from Middle High German s?, from Old High German) + Boot, boat.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.U-boat n (Nautical Terms) a German submarine, esp in World Wars I and II [from German U-Boot, abbreviation for Unterseeboot, literally: undersea boat]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014U-boat (?yu?bo?t) n. a German submarine. [1910?15;