to a T, . to a turn.

to a T,
Also, to a turn. Perfectly, exactly right, as in The description fitted him to a T, or The roast was done to a turn. The first expression, dating from the late 1600s, may allude to the T-square, used for accurate drawing, but some think it refers to crossing one's T's. The variant alludes to meat being turned on a spit until it is cooked to the proper degree. The variant was first recorded in 1780.

Done to a turn

Meaning

Cooked just right.

Origin

Since at least the end of the first Millennium, food, especially meat, has been cooked on spits. The English abbot and scholar, Aelfric of Eynsham, referred to them as 'spitu' in Latin Grammar and Glossary, circa 1000.

Spits were originally simple pointed sticks, which were used to hold meat near to a fire. Rotating spits were developed in the Middle Ages; initially turned by hand and later by various forms of powered mechanism.

The allusion in the phrase 'done to a turn', or 'roasted to a turn', is to food that had been cooked for the precisely correct number of turns of the spit. Both versions of the phrase date back to the 18th century and the 'roasted' form is first cited in a piece by an author called Mackenzie in Mirror No. 93, 1780:

"The beef was roasted to a turn."