Definition of swear by

The expression "swear by" has its roots in Old English and Anglo-Saxon culture. During that time, many people believed that certain objects or phrases had supernatural powers and could offer protection or good luck. In Old English, the phrase for swearing by such an item or belief was "swearthæg," which literally translates to "swear-thane." The word "thane" was commonly used in Old English as a translation for the Latin word "servus" or "servant." The use of the prefix "swear-" in "swearthæg" suggests that people took oaths or pledges in order to become a thane, or servant, of a particular item or belief. In essence, by swearing by something, they were pledging their loyalty and devotion to it. The word "swearthæg" evolved over time, eventually changing to "sweren bi" in Middle English. This phrase is the origin of the modern-day expression "swear by." In its current usage, swearing by something means to strongly and sincerely believe in its inherent value or efficacy, often as a way of seeking good luck or protection. The roots of this expression can be traced all the way back to the superstitions and beliefs of Old English society, providing a fascinating insight into the cultural and linguistic evolution of the English language.