Deeply ironic origin: the famous
相濡以沫 (cherishing each other through hard times) is the image Zhuangzi *rejects* in favor of
相忘于江湖. Modern Chinese frequently quotes both halves together to discuss friendship or love.
于 is classical 'in / at.'
江湖 literally 'rivers and lakes' — here the vast free waters, later 'the world-at-large.'