This word was derived from both middle english and contemporary jive talk. The -thu comes from the old "thou" which is, of course, "you". The -rr is added in modern english to complete the word. Altogether the word "thurr" means "I like your place right there", referring to a specific body part, usually the abdominal area of a female.
"I like yo ass right thurr, bitch!"
"Nice panties up thurr, ho"