Straight as a die

Die is the singular of dice, but from the late 1600s, a die was also a stamp or a mechanical device for shaping metals, coins etc. It is this meaning that is used in the expression 'true as a die', which was popular in the 1700s. 'Straight as a die' only appeared in the late 19th century, and even then the straightness referred to was originally honesty or integrity, rather than straightness as a linear dimension. Today the expression can be used for either.