The origin and etymology of the word tango is debated and researched even to this day.  There are several possibilities and perhaps all of these are in fact true.  In some African cultures the word tango can be found ( Mali, Angola) to mean  reserved ground or closed place. There is is also the latin root “ tangere” which means to touch, used in Portuguese and then passed to the slaves which were then imported  to Argentina. The word Tango appears to be a child of mixed blood the same as the dance itself.   It is almost certainly is a slang  term or became one as it traveled across cultures