To get the third degree means to be thoroughly
questioned. The third degree of something has been regarded as the
upper limit, or extreme since before the the time of Shakespeare
when he wrote, “For he’s in the thirde degree of drinke, he’s
drown’d.” He was referring to a very drunk man. It is a natural
progression when referring to the most extreme type of questioning,
it would be referred to as the third degree. It has come to also
mean inflicting of pain, physical or mental, to extract confessions
or statements.
In Masonic Lodges there is also the rituals involved before reaching
the third, or highest level, which includes intense questioning.