Here the allusion is to a ship grounded off a rocky coast, battered by waves and
ready to sink, and the expression arose among seamen, as would be expected. The
figurative use if rocks for a symbol of destruction or ruin dates back to at
least the early sixteenth century, but it wasn"t until three hundred years later
(1889) that the phrase on the rocks appeared or is first recorded. When we use
the expression today we always refer to ruin or impending disaster. Someone on
the rocks can be stone broke, or bereft of sanity. A marriage on the rocks is
wrecked and about to be sunk unless it is saved at the last minute. Even a drink
on the rocks (liquor or wine served over ice cubes, which resemble rocks) might
be said to be the cause of someone going under.