come hell or high water

Example: “Come hell or high water, we're finishing this house before winter.”

Origin: American, attested from the late 1800s, and pairing the worst imaginable spiritual outcome with a thoroughly practical one. The high water is the more revealing half: the phrase is commonly linked to cattle driving, where herds had to be pushed across rivers that could rise without warning, and a flooded crossing was the routine disaster standing between a drive and its destination. That association is plausible and often repeated but not firmly documented, and the phrase may simply have been built for its sound — the two stressed h's give it a hammering emphasis that a more literal formulation would lack. Either way the construction is the familiar one of naming two extremes to mean everything in between, and thus refusing every possible excuse in advance.