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The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Move toward or focus on a goal, as in He began with a couple of jokes before homing in on the main subject of his talk. This expression originally alluded to a vessel, aircraft or missile being guided to its target by a radio beam or some other means. [c. 1920]