educated guess, an - Phrases/Idioms
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.
A speculation based on past experience or knowledge, as in I'm not sure how much meat we need to feed twelve, but I'll make an educated guess and say six pounds. [Mid-1900s]