Giant was shy. He had considered professing his love for Giantess but she intimidated him. She was known to have killed one of her suitors for calling her generous. She looked down on all who called her beautiful. Someone must have also tried to woo her by describing her as ugly and miserly. She torched him alive. Such incidents deterred prospective suitors. Even the most flamboyant ones did not flirt with her shadow.
So one fine day, Giant donned a suit of armour and sent a passenger pigeon to Giantess with a note attached to the leg of the pigeon. Notably, this was the last known individual of that species. Considering it a rare gift, Giantess happily wolfed down the pigeon and was overwhelmed by Giant’s love. The note remained unread in the dark corner of Giantess’ stomach along with the hapless courier (that is, before the digestive juices started acting). However, for posterity’s sake, the note has been recorded:
You are sinfully beautiful, Giantess; neither ugly nor generous. When I see you, I feel a shot of adrenaline rushing through my veins, with my only thoughts being of fright and flight. Come to me, please, and deliver me from my agony by ensuring a life of perpetual plight. We will live in an abode of clouds, a heaven from where nothing must fall upon the earth. And I will love you till you tear me apart.
Much has been expounded from this note and the incidents that follow but we will stick to the tale. As you can probably guess, Giant and Giantess were married off and went to live in their cloudy home. For months they lived on without any casualty. But, as all stories take a bad turn, an element of strife needs to be introduced in this one too. So, one of these days, Giantess accused Giant of not washing the clothes properly. Apparently, Giant had gone downstairs to wash the clothes using the water from thunderclouds. Since everybody knows that water from thunderclouds is dark in colour, Giantess was worried about her white handkerchief turning a shade greyer. Now the timing of Giantess’ tantrum is crucial. All this happened while Giant was soaking beans. The one sided strife as a result of her wrath resulted in the ripping apart of Giant’s armour and more importantly the spilling of beans from the vessel.
The rest is history. Jack finds those beans and plants them to grow a giant beanstalk. He climbs the beanstalk, falls in love with Giantess and runs away with her, killing Giant in the process.
What I just narrated is the origin story of the idiom spilling the beans on somebody. In this particular case, the beans were spilled on to the old man who ultimately let the cat out of the bag by giving Jack those beans (in exchange for his cow). Hence, spilling the beans became synonymous with revealing a secret.
As the title suggests, you have to explain an idiom of your choice literally by concocting a story like this one. If anyone would like to write the origin story of letting the cat out of the bag by continuing from the thread above, he/she is welcome to do so.