When a dog carries its tail between its legs, it means the dog is feeling nervous, venerable, shameful or submissive. Dogs also put their tails between their legs when they are feeling frightened. It generally signifies that something is wrong, and it is a position that the dog takes around a dominant person or animal.
Dogs use their tails to express a variety of emotions, and some tail positions even express the dog's physical feelings. If a dog's tail is down, he may be feeling shy and trying not to be noticed. However, if his tail is arched upward, it signifies that he is feeling happy and welcoming.
If a dog's tail is down but not hidden between his legs, it usually means that the dog is not feeling well; it may signify an injury or a stomach ache.
Although a tail between the legs is usually considered a position of submissiveness, it is also fear-filled position. Dogs who strike this pose may be angry, and they may be getting ready to fight. Typically, if the dog wants to fight, he pairs the tail position with other signs of aggression such as growling, showing his teeth or settling back on his haunches as if preparing to lunge forward.