[ 'keɪs(ə)n , kə'su:n ]
■ noun
1》 a large watertight chamber in which underwater construction work may be carried out.
2》 a vessel or watertight structure used as a gate across the entrance of a dry dock or basin.
Origin
C17: from Fr., lit. 'large chest', from Ital. cassone , the spelling alt. in Fr. by assoc. with caisse 'case'.