The small, spherical particles made by rubbing boogers in a circular motion between the thumb and index finger. This is typically done after picking ones nose in order to discard the boogers (particularly when there is nothing on which to whipe your finger).
An important characteristic of greebles is that, after drying, they become hard. This gives them a scratchy or abrasive quality which makes them undesirable when deposited in bed sheets or on couch cushions.
Dude #1: Dude!? Quit getting your greebles in my car!
Dude #2: What do you want me to do, bro? Wipe it on my jeans?