What is fully canvassed?
A jacket should conform to the wearer's body. Therefore, a so-called interlining is attached to the facing of the jacket shoulders and the chest piece. The most common technique in modern readymade clothing is to attach the interlining with various types of adhesive. In classic traditional tailoring the interlining is basted (lightly sewn) by hand and later pressed to the customer's unique body shape. The technique of sewing the interlining is often called “fully canvassed” and is followed today by many exclusive suit manufacturers.
Footnote: Gieve & Co. later merged with military tailors Hawkes and became Gieves & Hawkes residing at No. 1 Savile Row. They still have genuine Reefer Jackets in their range. Italian Belvest wash all their interlinings before sewing them. This prevents the jacket from loosing its shape.