From Longman Dictionary of Contemporary Englishswear by something phrasal verb informalEFFECTIVEto have great confidence in how good or effective something is He swears by vitamin C pills. → swear→ See Verb table
Examples from the Corpus
swear by • The mother-goddess swore by a necklace of blue stones.• A: Some cooks say that basting does nothing to ensure moist meat; others swear by it.• There is no medical proof that taking Vitamin B6 helps, but some of his patients swear by it.• Other friends swore by Jenny Craig because it was so convenient.• The parson would swear by Jupiter to show he's a public school man but these old women would have him.• The Proconsul offers him a last-minute reprieve, however, if he will repent and swear by the genius of Caesar.• At the Morse School, principal James Coady and his teachers swear by the results.• Some swear by the techniques of Life Cycle Assessment, whilst others see it as a crude substitute for sound judgment. 