A close view of a U-boat.
U-boat definition by Webster's New World
Origin: ; from German U-boot, abbreviation, abbreviated of Unterseeboot, undersea boat
U-boat definition by American Heritage Dictionary
Origin: Translation of German U-Boot, short for Unterseeboot : unter, under (from Middle High German under, from Old High German untar; see ṇdher- in Indo-European roots) + See, sea (from Middle High German sē, from Old High German) + Boot, boat.