(someone's) heart is in (their) mouth

if someone's heart is in their mouth, they feel extremely nervous.

My heart was in my mouth as I walked onto the stage.

• a mystery to
• a mystery to you
• an about-face
• an accident waiting to happen
• an ace in the hole
• an Achilles' heel
• an acid test
• a nail in the coffin of
• a nail in the coffin of something
• an Aladdin's cave
• an all-rounder
• an amber gambler
• an ambulance chaser
• a name to conjure with
• An apple a day keeps the doctor away.
• an arm and a leg
• a narrow escape
• an article of faith
• an awkward customer
• and a half
• and all
• and all that jazz
• and hang the cost
• and hang the expense
• And how!