My favorite definition so far.  It brings to mind the story about the
two navvies down in a ditch digging while the supervisor sits under a
tree in the shade.  One says to the other, why is this?!  The other
says, "I'll go ask."  He climbs out of the ditch and asks the
supervisor.  "It's like this," says the super.  The super holds a shovel
upright in front of the tree.  "Hit the shovel with your fist." The
navvy swings to hit the shovel and the super moves the shovel aside so
that the poor fellow drives his fist into the tree.  He returns to the
ditch.  "What did he say?" asks his friend.  "Well, he said it's like
this,"  and not having a tree, holds the shovel in front of his face...