This word can mean all sorts of things depending on the context, particularly in America. However, tank as in the military vehicle, is of British origin and dates from 1915. When the first tanks were shipped by rail to the Western Front in December 1915, they were marked as water tanks as a cover name for security reasons and the name tank stuck thereafter. Tank, as in a detention or holding cell, dates from the early 1900s and is American in origin. To tank a game is also an American expression meaning to lose a game deliberately and dates from the 1970s. This meaning originates from boxing from the early 1900s when ‘to go in the tank’ was US slang for ‘to take a dive’, which was even more slang for losing a fight deliberately by taking a dive to the canvas. This meaning of tank probably has associations with clowns being deliberately dumped into tanks of water at fairs and circuses. From the 1980s, tank meaning to lose a game deliberately migrated to mean a sharp decline or fall, as in “the share price or the market tanked”. Somewhat confusingly, in the 1990s, to be in the tank for someone meant to give them one’s full support and this meaning seems to be exclusively American. See also Leave something in the tank.