The image here is of a dog wagging it’s tail as an expression of happiness.
RELATED IDIOMS :
dog and pony show
an elaborate display or performance designed to attract people's attention - North American informal
1998 - Spectator - Happy as I always am to help the Bank of England, I have supplied the script for its EURO dog and pony show.
dog eat dog
a situation of fierce competition in which people are willing to harm each other in order to succeed
This expression makes reference to the proverb dog does not eat dog which dates back to the mid 16th century in English and before that to Latin canis caninam non est a dog does not eat dog's flesh.
1998 - Rebecca Ray - A Certain Age - It's dog eat dog, it's every man for himself. Right from the start, fighting amongst ourselves for the few decent wages left.
dog in the manger
a person inclined to prevent others from having or using things that they do not want or need themselves.
This expression comes from the fable of the dog that lay in a manger to prevent the ox and horse from eating the hay.
the dog's bollocks
the best person or thing of its kind - British vulgar slang
a dog's dinner = a dog's breakfast
a poor piece of work
a mess - British informal
The image is of a dog's meal of jumbled-up scraps.
2000 - Independent - He was rightly sacked because he had made such a dog's dinner of an important job.
a dog's life
an unhappy existence full of problems or unfair treatment
1987 - Fannie Flagg - Fried Green Tomatoes at the Whistle Stop Cafe - The judge's daughter had just died a couple of weeks ago, old before her time and living a dog's life on the outskirts of town.
utterly worn out – informal
The image here and in the variant dog weary is of a dog exhausted after a long chase or hunt.
dogs of war
the havoc accompanying military conflict - literary
This phrase is from Shakespeare's Julius Caesar - let slip the dogs of war. The image is of hunting dogs being loosed from their leashes to pursue their prey.
1998 – Times - The good guys may have broken the rules by employing dogs of war.
dressed like a dog's dinner = dressed up like a dog's dinner
wearing ridiculously smart or ostentatious clothes - British informal
every dog has his day = every dog has its day
everyone will have good luck or success at some point in their lives – proverb
give a dog a bad name
it is very difficult to lose a bad reputation, even if it is unjustified
This is a shortened version of the proverb give a dog a bad name and hang him which was known from the early 18th century.
go to the dogs
deteriorate shockingly, especially in behaviour or morals – informal
This idiom derives from the fact that attending greyhound races was once thought likely to expose a person to moral danger and the risk of incurring great financial loss.
1997 - Daily Telegraph - If you read the English media or watch the cretinosities of television, you would think that the country is going to the dogs.
the hair of the dog
a small quantity of alcohol taken as a remedy for a hangover – informal
The full form of this phrase is hair of the dog that bit you. Hair from a rabid dog was at one time thought to be a remedy against the effects of its bite. In this expression, the recommended cure for a hangover is a small amount of the cause of the problem.
1987 - Bruce Allen Powe - The Ice Eaters Murray, still feeling the effects of the previous evening, had suggested they go into a bar because he needed a hair of the dog.
help a lame dog over a stile
come to the aid of a person in need
in a dog's age
in a very long time – North American informal
keep a dog and bark yourself
pay someone to work for you and then do the work yourself.
1991 - Purchasing and Supply Management – He does not solve the subcontractor's technical problems, keeping a dog and barking himself.
let the dog see the rabbit
let someone get on with work they are ready and waiting to do – informal
This phrase comes from greyhound racing where the dogs chase a mechanical rabbit around a track.
let sleeping dogs lie
avoid interfering in a situation that is currently causing no problems, but may well do so as a consequence of such interference – proverb
In the early 14th century the French phrase n'esveillez pas lou chien qui dort advised do not wake the sleeping dog while Chaucer remarks in Troilus and Criseyde…it is nought good a slepyng hound to wake. The present form of the proverb seems to be traceable to Walter Scott's novel Redgauntlet (1824).
not a dog's chance
no chance at all
put on the dog
behave in a pretentious or ostentatious way - North American informal
Dog was late 19th-century US slang for style or a flashy display.
1962 - Anthony Gilbert - No Dust in the Attic - Matron put on a lot of dog about the hospital's responsibility.
rain cats and dogs
rain very hard
Despite much speculation, there is no consensus as to the origin of rain cats and dogs. Suggestions range from the supernatural (cats being associated with witches who were credited with raising storms, dogs being attendants upon Odin - the Scandinavian storm god) to the down-to-earth (animals in medieval times drowning in flooded streets in times of heavy rain and their bodies being assumed by the credulous to have fallen from the skies). Other versions of the saying are rain pitchforks and, in Britain, rain stair rods, which date from the early 19th century and mid 20th century respectively and reflect the shaft-like appearance of heavy rain. Rain cats and dogs is first recorded in Jonathan Swift's Polite Conversation (1738).
sick as a dog
extremely ill - Informal
throw someone to the dogs
discard someone as worthless
you cannot teach an old dog new tricks
you cannot make people change their ways - proverb