être dans de beaux draps

To be in serious trouble

Said ironically. Being in 'fine sheets' is actually a right mess to be in.

Si le patron découvre ça, on est dans de beaux draps.

If the boss finds out about this, we're in serious trouble.

The 'sheets' here were the white garments people once had to wear in public as penance for a wrongdoing. Over time the phrase kept its ironic edge and came to mean any tricky or compromising spot.