Keep a lid on

From Fresh Dictionary

English

Transitive phrasal verb

keep a lid on (often with -it)

1. ((idiomatic{{#if:|, {{{2}}}{{#if:|, {{{3}}}{{#if:|, {{{4}}}{{#if:|, {{{5}}}{{#if:|, {{{6}}}{{#if:|, {{{7}}}{{#if:|, {{{8}}}{{#if:|, {{{9}}}}}}}}}}}}}}}}}}})) to keep something secret

   It's supposed to be a surprise, so try to keep a lid on it, will you?

See also

• under wraps