Hell or high water
Any great difficulty or obstacle.
The derivation of this phrase isn't well-understood. It doesn't appear to allude to any particular thing or event. It it most probably just an impressive-sounding alliterative phrase that refers to things that are obviously difficult to overcome. It is American and appears in many U. S. sources before the first citation elsewhere - which isn't until 1915.
The earliest American reference I can find is from the Iowa newspaper The Burlington Weekly Hawk Eye, from May 1882. This piece, in what Mel Brooks might call 'authentic frontier gibberish', is a reprint from 'The Little Rock Gazette'.
"Since dat time de best ob my friends hab become enemies, an' strangers hab become friends. De debil had brook loose in many parts ob de country, an' keepin' up wid de ole sayin', we've had unrevised hell and high water - an'a mighty heap ob high-water I tell yer."
To be rated as on old saying in 1882 we can surmise that it dates back until at least the mid-19th century.