SouthernCelt, the old man's saying "___ ain't twissed tight" is a variant form of what I believe to be a very common African American saying that [someone] isn't {aint} wrapped too tight".

Another way of saying this is that "_____ isn't playing with a full deck" or "____ doesn't have all his wits about him."

The implication of a person not being "twisted tight" or not "being wrapped too tight" is that he or she has probably more than one brain "screw[s] loose".