From: http://www.bbc.co.uk/worldservice/learningenglish/language/2013/11/131114_todays_phrase_hold_your_tongue.shtml

Hold your tongue

A chameleon extending its tongue to catch a cricket
A chameleon extending its tongue to catch a cricket. Photo: William West/ AFP/ Getty Images

Today's Phrase

If someone tells you to hold your tongue it means they want you to stop talking, because they don't like what you are saying.
Examples:
I'm going to have to hold my tongue because I keep saying what I think and people get upset!
I had to tell Jane to hold her tongue; what she was saying didn't help us move forward in our planning meeting.

Take note

If you make a slip of the tongue, you pronounce a word incorrectly or you say something you didn't mean to say.
Example:
I meant to tell her she was fun not fat; it was just a slip of the tongue!

Interesting fact

Chameleons, like the one in this photo, are able to change colour, though not all species can. In a few types, the colour change is used as camouflage, but in most it is a form of communication to attract potential mates or warn off rivals. They will revert to a default colour that merges with their habitat - green for a jungle species, beige for a desert one.