bite (your) tongue

to stop yourself from saying something because it would be better not to, even if you would like to say it.

I really wanted to tell her what I thought of him but I had to bite my tongue.

• first and foremost
• First come, first served.
• first-come, first-served
• first dibs on
• first dibs on something
• first-hand
• first hand
• [first/highest/next etc.] rung on the ladder
• First in, best dressed.
• first-past-the-post
• first rung on the ladder
• first string
• first-string
• first thing
• first thing in the morning
• first things first
• First things first.
• fish for compliments
• fishing expedition
• fish or cut bait
• Fish or cut bait.
• fish out of water
• fit in
• fit in someone
• fit in something