Rather than the thumbs up or thumbs down response to something, "thumbs in" denotes an inability to care less. While holding one thumb up and the other thumb down, you then bring both thumb gestures behind you and motion to place both thumbs "in" your ass.

One of your five fingers. That's right! It's a FINGER. The thumb is a finger, so fuck you assholes who say you have four fingers and a thumb!

The hip new way to indicate you are texting. Matter of fact; all the really cool kids now refer to texting as "thumbing."

when someone shoves their thumb into a pleasurable area.

placing your thumb into ones crevice.

When fucking and you get brewers droop or can't manage anything more than a slack-on, one is required to 'thumb it in'
As seen on many porno's

Used to describe the appearance of a very obese person whose face and double chin have merged into one unit.