We actually only know this term from the emotional or mental realm. That is, when someone is overwhelmed by their feelings. But originally it had a different meaning.The term comes from the middle english word whelven, which means to turn upside down, a vessel is said to be overwhelmed when she has capsized or has turned upside down in the water. 

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In the 17th century, the term capsized appeared more and more, until it finally replaced overwhelmed in nautical terms and this term slipped into the emotional world.