marsh

Marsh comes from Old English mersċ, from Proto-West Germanic *marisk, which was built from *mari ("sea, body of water") plus the suffix *-isk, meaning roughly "like the sea." That root *mari traces back further to Proto-Germanic *mari and Proto-Indo-European *móri.

The word has stayed remarkably steady in meaning — from Old English mersċ to Middle English mersch to modern marsh, it has always pointed to wet, sea-adjacent ground.

It's a doublet of marish, morass, and merse, and cognate with West Frisian mersk, Dutch meers and moeras, and German Marsch. It's also tied to mere, since -ish was simply added onto that word for "sea, body of water."