Everything that is relevant; the whole thing. For example, He decided to take everything to college--his books, his stereo, his computer, his skis, the whole nine yards. The source of this expression is not known, but there are several possibilities: the amount of cloth required to make a complete suit of clothes; the fully set sails of a three-masted ship where each mast carries three yards, that is, spars, to support the sails; or the amount of cement (in cubic yards) contained in a cement mixer for a big construction job. [Colloquial]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.