The term GERMAN SUB is a reference to two types of German submarines used during World War II. The first is ERSATZ, which is a 6-letter word meaning 'SUBSTITUTE' or 'REPLACEMENT'. This type of submarine was used as a training vessel for the German Navy. The second is UBOAT, a 5-letter word meaning 'undersea boat'. This type of submarine was used for combat operations and was responsible for sinking hundreds of Allied ships during the war.