swear by

swear by idiom
  1. Have great reliance on or confidence in, as in She swears by her personal physician. [Early 1800s]
  2. Also, swear to. Have reliable knowledge of, be sure of, as in I think she was going to the library but I can't swear to it. [Mid-1700s]
  3. Take an oath by, as in I swear by all the saints in heaven. [Early 1200s]

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.

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