A literal security blanket. The term was made popular by its use in the 1983 movie Mr. Mom.
Now listen to me, I understand that you little guys start out with your wubby's, and you think they are great...and they are. They are terrific, but pretty soon a wubby isn't enough. You're out on the street trying to score an electric blanket, maybe even a quilt. Then the next thing you know your strung out on bedspreads, Ken! That's serious! Now gimmie the wubby.