Fig.: to be extremely nervous or worried

Finding himself alone and lost in the abandoned mine, he broke out in a cold sweat as he searched for a way out

Resosi conto di essersi perso e di essere solo nella miniera abbandonata, cominciò a sudare freddo e si mise a cercare una via di uscita

Tom thinks he's as good as a Formula 1 driver, but he's just a reckless driver. I sweat blood every time I ride with him

Tom crede di essere bravo quanto un pilota di Formula 1, ma in realtà è solo un guidatore incosciente e sudo freddo ogni volta che salgo in macchina con lu

Being afraid of heights, Andrew sweated bullets when he had to go on the roof to check the chimney

Andrea soffre di vertigini e quando è dovuto andare sul tetto per controllare il camino, sudava freddo

The origin of this expression is unclear. One theory relates it to the older phrase to sweat blood, which refers to Christ's agony in the Garden of Gethsemane: "And being in agony He prayed more earnestly; and His sweat became like great drops of blood falling down upon the ground" (Luke 22:44).
Another theory suggest that the bullets in question are simply very large drops of sweat, their size and shape exaggerated and likened to that of bullets, thus emphasizing the distress of the person under pressure