Beyond the literal 'bottom of the box',
箱底 carries two metaphorical layers: a
压箱底 ('pressing the bottom of the chest') item is a household's most precious possession, kept buried under everything else; and
箱底 can also stand for a secret you tuck away. The verb
压箱底 is now widely used for 'best-kept' anything - a chef's signature dish, an actor's career-best work.
