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.