ground floor, get in on the
Participate in the beginning of a venture, thereby gaining some advantage. For example, Investors were eager to get in on the ground floor of the new development. [First half of 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.