The plural or alternative form of athwartship, used the same way to describe sideways movement across a vessel.
Variant plural form of 'athwartship,' maintaining the same nautical origin with Old English roots in 'athwart' and 'ship,' used interchangeably in maritime English.
Maritime language evolved multiple acceptable forms for the same concept because sailors working together needed flexibility in speech—'athwartships' and 'athwartship' both survive in nautical writing because both worked fine in the chaos of sailing ships.
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