1. a ditch or moat, especially one used for fortification
Usage: archaic; chiefly British
2. a waterfall or cascade
Usage: dialectal; Northern English

Examples

• The ancient castle was protected by a deep foss filled with water.
• Medieval fortifications often included a foss as the first line of defense.
• The old foss around the manor house had long since dried up.
• Tourists gathered to admire the spectacular foss tumbling down the cliff.
• The hiking trail led to a hidden foss in the Yorkshire Dales.