In the engineering context, dog knot may mean the enlarged section of a rod or pipe that is used to retain a hole. These dog knot structures are found inside buildings and the likes. This engineering definition of dog knot is not really popular. As such, it is rarely used even within the industry. But then again, this is the initial meaning of the term. It has, however, evolved greatly and at times, quite inappropriately.
A dog knot is also a type of knot tied round a shaft. This is done so that the rope attached to the structure won't come loose. If you are an expert in making knots, then you should know how to make this particular knot. The knot is quite useful, especially if you need to tie two objects together with a string or rope.
However, as far as dog knot is concerned, people highly associate it to sex nowadays. The term actually came from two dogs mating. Dogs mate with each other during the bitch's heat season and when the act do occur, it is next to impossible to take the two pets apart from each other. They are bound together by male dog's reproductive organ, thus causing the dog knot as it is referred to.
Dog knots occur when the dog is involved in an intercourse because its penis inflates. Since the penis becomes bigger inside the bitch's body after penetration, it becomes stop the act. This is the reason why you can't take two dogs mating with each other apart. This is also the reason why the act is referred to as dog knot by breeders and some pet owners.
Over time, the dog knot concept became applicable in humans too. A lot of perverts tried to adopt dog knot to their sexual actions. The main idea is to be attached to a woman in the same was as two dogs are welded together. Dogs mate either back-to-back or with the male dog in standing position behind the female dog. This is also the position that some partners assume as they try to apply the dog knot concept in their intercourse. This definition of dog knot is part of the colloquial or slang dictionary and is never used in formal writing.
There's yet another dog knot definition that has to be explored. Dog knot may also pertain to the movement or waste of a dog. Dog wastes are similar to a pretzel in form. And so, many pet owners refer to their dog's fritter as the dog knot. Again, this dog knot definition is considered slang.
Simply put, the real meaning of this term would depend entirely on how it was used. With the different definition of dog knot, it is always upon the speaker to associate the correct words to the term to express the right idea.