seal的意思是“封闭，密封”。从字面上看one's lips are sealed是“某人的嘴唇被密封”，实际上，该短语的真正意思是指“口风紧；守口如瓶”。

【例句】
You can trust me with your secret. My lips are sealed.
你可以放心地告诉我你的秘密。我一定会守口如瓶。

I told him not to say anything, and he said his lips were sealed.
我告诉他不要把任何事情说出去，而他也表示他会守口如瓶。