I had always accepted that this idiom is used when referring to something that is too small to make a difference. I was surprised when I looked up the origin. The Phrase Finder writes that it was used in the Bible, Isaiah 40:15, “Behold the nations are as a drop of a bucket, and are counted as the small dust of the balance…” “A “drop in the bucket’ is the predecessor of “drop in the ocean,” which was first published in The Edinburgh Weekly Journal in July 1802.