Same physical/figurative split as English 'opaque'. Often used metaphorically about governments, contracts, or pricing — 不透明的决策 (bú tòumíng de juécè) 'opaque decision-making'. Antonym: 透明 (tòumíng) 'transparent'.
memory
透 (tòu) 'penetrate/through' + 明 (míng) 'light/bright' = '(allow) light through' = transparent. Add 不 (bù) 'not' in front and you've negated it.
