An endorsement of something or someone, as in Our candidate doesn't have the governor's seal of approval, or The new management gave the old refund policy their seal of approval. This idiom was used, and perhaps invented, as an advertising gimmick of Good Housekeeping Magazine, which gave its so-called “seal of approval” to products it endorsed; the products' packaging in turn bore a small emblem attesting to this endorsement. The noun seal here is used in the same sense as in set one's seal on.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.