The spelling of the word "theloncus" may seem confusing to some, but it can be explained using phonetic transcription. This word is pronounced /θɛlɒŋkəs/, with the first syllable being pronounced like "thel" with a soft "th" sound. The second syllable has a short "o" sound like "lot". The third syllable has a hard "ng" sound like in "sing". The final syllable has a soft "s" sound at the end. Despite its unusual spelling, this word can easily be pronounced once one understands its phonetic components.
A tumor of the nipple.
A practical medical dictionary. By Stedman, Thomas Lathrop. Published 1920.