Etymology
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tank top (n.)
1968, from tank suit "one-piece bathing costume" (1920s), so called because it was worn in a swimming tank (n.), i.e. pool.
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tank (v.)

1900, "to put into a tank," from tank (n.). Meaning "to lose or fail" attested from 1976 in a general sense, apparently 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.

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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.

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top (adj.)

"being at the top," 1590s, from top (n.1). Top dollar "high price" is from 1942. Top-drawer (1920) is from British expression out of the top drawer "upper-class." Top ten in popular music is from 1945 ("Billboard"). The top dog is the one uppermost in a fight, from 1868 in figurative use, opposed to the underdog.

But if the under dog in the social fight runs away with a bone in violation of superior force, the top dog runs after him bellowing, "Thou shalt not steal," and all the other top dogs unite in bellowing, "This is divine law and not dog law;" the verdict of the top dog so far as law, religion, and other forms of brute force are concerned settles the question. [Van Buren Denslow, "Modern Thinkers: What They Think and Why," 1880]
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top (n.1)

"highest point," Old English top "summit, crest, tuft," from Proto-Germanic *toppa- (source also of Old Norse toppr "tuft of hair," Old Frisian top "tuft," Old Dutch topp, Dutch top, Old High German zopf "end, tip, tuft of hair," German Zopf "tuft of hair"); no certain connections outside Germanic except a few Romanic words probably borrowed from Germanic.

Few Indo-European languages have a word so generic, which can be used of the upper part or surface of just about anything. More typical is German, which has Spitze for sharp peaks (mountains), oberfläche for the upper surface of flat things (such as a table). Meaning "highest position" is from 1620s; meaning "best part" is from 1660s. To go over the top is World War I slang for "start an attack," in reference to the top of the trenches; as "beyond reasonable limits, too far" it is recorded from 1968. Top of the world as "position of greatest eminence" is from 1670s. Top-of-the-line (adj.) is by 1950.

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top (n.2)
"toy that spins on a point," late Old English top, probably a special use of top (n.1), but the modern word is perhaps via Old French topet, which is from or influenced by a Germanic source akin to the root of English top (n.1). As a type of seashell, first recorded 1680s.
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top (v.)
"put a top on," 1580s, perhaps mid-15c., from top (n.1). Earlier "cut the top off, shave the head" (c. 1300). The meaning "be higher or greater than" also is first recorded 1580s. Meaning "strike (a ball) towards its top" is from 1881. Related: Topped; topping. To top off "to finish" is colloquial from 1836; in sense "fill up, add more to to bring to fullness" it is from 1917.
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fish-tank (n.)
1921 as an ornamental object, from fish (n.) + tank (n.).
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oil-tank (n.)

"tank for holding oil," 1862, from oil (n.) + tank (n.).

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box-top (n.)
1937, American English, from box (n.1) + top (n.1).
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