The spelling of the word "swith" is peculiar because it is not commonly used. The word means "quickly" or "suddenly," and its pronunciation is /swɪθ/. The "th" sound is spelled with the letter "th" but pronounced like "th" in "then" rather than "th" in "thin." The "sw" is pronounced like "s" followed by "w." While the spelling is not intuitive, it is important to learn uncommon words and their correct pronunciations to enhance one's vocabulary.
The word "swith" comes from the Old English word "swīþ", which means "strong" or "powerful". This term is derived from the Proto-Germanic word "*swīþaz", which has the same meaning. It is related to other Germanic languages such as Old High German "swid", Old Norse "svinnr", and Dutch "sterk". The word has evolved over time to its present-day usage, where it mainly appears in certain dialects of English, particularly in Scotland and the North of England.