This phrase means to hide or control something in order to prevent people from finding out about it. In a figurative sense one 'keeps the lid' on important information when it could give aid and comfort to the enemy. It means keeping it quiet, out of sight and out of hearing, it shouldn't be let out (open to public knowledge) just yet.

Übersetzung:

etwas streng unter Kontrolle halten

Gegenteil(e)

The opposite of "keep the lid on something" is "to take the lid off something".

Synonym(e):

You could also say: to maintain control over something.