straight as a die

Etymology

 * Used in the sense of ‘correct,’ attested since April, 1871, originally American English, from the 18th-century expression, from the 16th-century expression , referring to the osseous substance that dice were formerly created with.
 * Possibly connected with, or confused with, the metal-cutting tool called a.

Adjective

 * 1)  Completely or totally straight.