Language does not change in a vacuum and can get modified by an almost immeasurable amount of factors. From generation to generation new words are borrowed or invented.
The meaning of old words can drift dramatically from one group of language users to the other. This is a case of two definitions of the same word being so dramatically different between two groups that they end up clashing.
One of those groups, however, does not want to let the other's definition stand and is frequently insulted by the mere mention of it.