tank (n.)
1610s, "pool or lake for irrigation or drinking water," a word originally brought by the Portuguese from India, from a Hindi source, such as Gujarati tankh "cistern, underground reservoir for water," Marathi tanken, or tanka "reservoir of water, tank." Perhaps ultimately from Sanskrit tadaga-m "pond, lake pool," and reinforced in later sense of "large artificial container for liquid" (1680s) by Portuguese tanque "reservoir," from estancar "hold back a current of water," from Vulgar Latin *stanticare (see stanch). But other sources say the Portuguese word is the source of the Indian ones. Meaning "fuel container" is recorded from 1902. Slang meaning "detention cell" is from 1912. Railroad tank-car is from 1874.

In military use, "armored, gun-mounted vehicle moving on continuous articulated tracks," the word originated late 1915. In "Tanks in the Great War" [1920], Brevet Col. J.F.C. Fuller quotes a memorandum of the Committee of Imperial Defence dated Dec. 24, 1915, recommending the proposed "caterpillar machine-gun destroyer" machines be entrusted to an organization "which, for secrecy, shall be called the 'Tank Supply Committee,' ..." In a footnote, Fuller writes, "This is the first appearance of the word 'tank' in the history of the machine." He writes that "cistern" and "reservoir" also were put forth as possible cover names, "all of which were applicable to the steel-like structure of the machines in the early stages of manufacture. Because it was less clumsy and monosyllabic, the name 'tank' was decided on." They were first used in action at Pozieres ridge, on the Western Front, Sept. 15, 1916, and the name was quickly picked up by the soldiers. Tank-trap attested from 1920.
tank (v.)
1900, "to put into a tank," from tank (n.). Meaning "to lose or fail" attested from 1976, originally in tennis jargon, specifically in an interview with Billie Jean King in "Life" magazine, Sept. 22, 1967:
"When our men don't feel like trying," she says, "They 'tank' [give up]. I never tanked a match in my life and I never saw a girl do it. The men do it all the time in minor tournaments when they don't feel like hustling. You have to be horribly competitive to win in big-time tennis."
Sometimes said to be from boxing, in some sense, perhaps from the notion of "taking a dive," but evidence for this is wanting. Related: Tanked; tanking. Adjective tanked "drunk" is from 1893.