one and only

One's only love; one's only sweetheart. For example, He swore she was his one and only, but the detective following him knew better. [c. 1900]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.