avere la coda di paglia

Literally: To have a straw tail

Example: “Si è agitato appena ho nominato le chiavi: ha la coda di paglia.” — He got nervous as soon as I mentioned the keys; he clearly has something to hide.

Origin: The most commonly repeated explanation credits an Aesop's fable about a fox who, having lost her own tail, was so ashamed that she fashioned a replacement out of straw — first documented as the phrase's source by the Sienese writer Temistocle Gradi in 1865, then repeated by lexicographer Pietro Fanfani in 1870, which is largely how it became the standard accepted origin. The theory has a real problem, though: no actual version of Aesop's fox-and-tail fable has the fox wearing a straw substitute at all, making the connection considerably shakier than its wide acceptance suggests. A second, less romantic theory reaches for genuine medieval practice: convicted criminals or defeated combatants were sometimes made to wear an actual tail of straw attached to their clothing as a public mark of shame and humiliation, visible proof of guilt that made the wearer understandably anxious around anything that might expose them further — fire being the most literal danger, since straw catches alight easily.