Let us first flush out any misconceptions about what this word comes from or what it might mean. Cistern is not a seabird (tern) that has stayed on the same side (cis) of the road, the pond, or whatever (why would it do that anyway, instead of crossing? I guess because there’s a fellow tern there, and one good tern deserves another). Cistern is not, though it sounds like it could be, a converse to brethren (in fact that’s sistren). And it is not a medieval stringed musical instrument (that’s a cittern). Any proposal that it is related to those is thus tanked.
I presume you know what a cistern is: it’s a reservoir for water – not a cup, not a jug, a proper fixed container of substantial size. It may be a tank for catching and holding rainwater; it may be the large reservoir at the top of your (and my) building that holds the drinking water supply (why up there? water pressure depends on the height of the vertical column – that’s why so many towns have water towers rather than just ground-level reservoirs); it may be the vessel surrounding the condenser on a steam engine; it may be the tank on your toilet – well, if you’re in England, anyway. I’ve never heard it used as such in North America, and was first made aware of the cultural difference in a bit of Marxist graffiti from England (in a book, collected by Nigel Rees) where someone had written on a water closet (WC), “Man is born free but everywhere is in chains. Smash the cistern.” (I didn’t get the pun at first either.)
So where did this word come from? English got it from Old French cisterne; Old French got it from Latin cisterna; that was formed from Latin cista (‘trunk, chest, casket’); that in turn came from Greek κίστη kístē (‘box, chest, casket’). And κίστη also, at length (and via Proto-West-Germanic), transformed into English chest.
But we are in general oblivious to the deeper histories of the words we use. No one is likely to see cistern and think of a box or chest that doesn’t hold water. And similarly, though words can have echoes of other words (which can also be useful in puns from time to time), you won’t hear tern or stern and think of a tank (though if you hear sternum you might think of a tank top – “tanks for the mammaries” so to speak). Ultimately, cistern is now, in the main, just a fancier, perhaps thirstier-sounding word for tank.
Not for just any tank, of course; only the kind that is filled from rain or a well. Tanks are also military vehicles, so named because they looked at first like large rolling cisterns. I am reminded of Sam Peckinpah’s 1977 war movie Cross of Iron, in which a soldier, seeing such vehicles approaching, screams “Tanks! Tanks!” In the French version, the translator did not render this as “Chars d’assaut! Chars d’assaut!” (the standard French term) nor as “Citernes! Citernes!” (the direct translation of the word). No, it came out as “Merci, merci!”
Well. You’re welcome.