swear by

Idioms  

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]

---

Example Sentences

“At the end of the day, the government is going to go with systems that have been deployed, where combatant commanders swear by the fact that they want one of what we have,” Mengucci said.

From Barron's • Jan. 15, 2026

Lavender sleep masks, massages and mind tricks involving socks: Jet-setting bosses swear by these hacks that they say don’t require flying business class.

From The Wall Street Journal • Jan. 11, 2026

For some people it's a reward after exercise while for others, it's the main attraction and many regulars swear by it, convinced that 15 minutes of intense heat does wonders for their body and mind.

From BBC • Dec. 29, 2025

Some people swear by writing a song every day and finishing it, even if it’s bad.

From Los Angeles Times • Dec. 1, 2025

“Sparra word always good. Warbeak no say lie. Me swear by mother’s egg. That big swear.”

From "Redwall" by Brian Jacques