word usage - Is it 'thumb is up' in 'thumbs up'?

It was a thumbs up on the new filtration plant at Thursday's village board meeting

This is so common but then I never thought too deep unless I became a fan of English language after joining this site!

• 
  
  

  Is thumbs up actually (your) thumb is up? But then why thumbs? And not thumb's up - thumb is up (for this matter)?

  
  


• 
  

  Is thumbs up actually everybody's thumbs up? But then why not thumbs're up?

  
  


• In above-mentioned case, it says a thumbs up. The article a?

Okay, while doing thumb(s)-up, you don't do it with both the thumbs but with one. So, it's actually thumb's up!