What does "pull the plug on" mean?

Discontinue, end, as in The government pulled the plug on that program. [First half of 1900s]

Remove all life-supporting equipment, as in The family debated whether it was time to pull the plug on him. [Second half of 1900s] Although this idiom undoubtedly alludes to cutting off electricity to an electrical device, it originally referred to the removal of a stopper that flushed an old-style toilet.