The phrase "wast in session" is an archaic way of expressing that someone or something was currently in session. The word "wast" is an old English verb form of the word "was" and has fallen out of use in modern English. The correct pronunciation of "wast" is /wɑːst/ - the "a" sound is pronounced as "ah" and the "st" ending is pronounced as a soft "s" sound. Although the spelling may seem unusual, it reflects the historical evolution of the English language.