stanch (v.)

1660s, "cease to run or flow, be or become stagnant, stand without current," from Latin stagnatum, stagnatus, past participle of stagnare "to stagnate," from stagnatum "standing water, pond, swamp," perhaps from a PIE root *stag- "to seep drip" (source also of Greek stazein "to ooze, drip;" see stalactite). The figurative sense of "cease to be brisk or active" is by 1709. Related: Stagnated; stagnating.

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 it is ultimately from Sanskrit tadaga-m "pond, lake pool."

The meaning "large artificial container for liquid" (in English by 1680s) might have been reinforced by Portuguese tanque "reservoir," from estancar "hold back a current of water," from Vulgar Latin *stanticare (see stanch). But Century Dictionary and other sources say the Portuguese word is the source of the English one and the resemblance to the Indian words is coincidental. 

The meaning "fuel container of a motor vehicle" is recorded from 1902. The slang meaning "detention cell" is from 1912. The railroad tank-car, for bulk transport of liquids, is from 1874.