Summary:

Shoo-in first appeared in the 1930s, referring to horse racing and its corrupt practices, including rigged outcomes.  Shoo means driving something away, like shoo-fly, meaning “don’t bother me, I belong to company G.  “Shoo-fly was a song recorded in 1889.  At the same time, a Shoo-in suggests that someone or something is sure to win.

https://en.wikipedia.org/wiki/Shoo_Fly,_Don%27t_Bother_Me

https://allnurseryrhymes.com/shoo-fly-dont-bother-me/

Shoo-in is widely used in politics today to suggest the positive possibility of winning.

Article:

“Hooing’’ is an action that involves driving something or someone away, often used to instill fear or intimidation.  This concept of ‘shooing’ forms the basis for the term ‘shoo-in’.

Examples of usage of shoo-in:

When the boss croaks, I am a shoo-in to take over.

Orange County, California, is a shoo-in to earn the reputation of the best place to live in California.

I am backing the XYZ candidate for mayor of Dana Point, California.  She is a Shoo-in to win.

Sacramento has a 94% registered Democratic voter base in its district. Whatever the Democrats want, they are a shoo-in to get.

Liberal politicians in California appear to be a shoo-in when they run against conservatives.  The question is, why?

You may have a little fun with your friends, suggesting that when you compare your intellect to theirs, you are a shoo-in winner.