(Ato wa no to nare yama to nare;
“After this, come field, come mountain.”)
Not caring about what comes next as long as the current problem or issue can be finished up or taken care of. By extension, not caring about what happens to anybody else as long as you can get what you want. “After me, the deluge.” The image is of taking care of a piece of land, but not caring what happens to it afterwards – even if it reverts to a wild field or somehow turns into a mountain!
We begin with the noun 後 (ato), “after,” marked as the topic of discussion by the particle は (wa). Next we have a repeated verb phrase with different nouns. The verb is なる (naru), “to become.” In each case, the verb relates to the preceding noun through the particle と (to), which in this case marks the endpoint of a change. The first potential becoming is 野 (no), “field,” and the second is 山 (yama), “mountain.”
The grammar of the nares isn’t 100% clear, but I suspect that the verb is in perfective form, which allows it to take the (elided but implied) suffix ば (ba), “(even) if.”
This saying apparently comes to us from 冥土の飛脚 (Meido no hikyaku), The Courier for Hell, an early 18th-century joururi love-suicide play by Chikamatsu Monzaemon. And yet its relevance to current events is almost too obvious.
This kotowaza is specifically noted as being antonymous to 立つ鳥跡を濁さず.
(“Ato wa no to nare yama to nare to bakari ni zeishuunyuu wo waza to herashite yagatte, aitsura maji de nani kangaeteru ndarou.” “Sore yori, tada issai mo kangaeteinai to itte iin ja nai?”)
[“Those idiots are slashing tax revenues on purpose as if they don’t care what comes next. What on earth are they thinking?” “Isn’t it just that they’re not thinking at all?”]