The far-reaching power of the authorities. For example, You'll never get away with leaving work early; the long arm of the law is bound to catch you. This expression began as Kings have long arms (or hands) and was listed as a proverb in 1539. The current version, now often used lightly, was first recorded in 1908.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.