A Yiddish word for a female gossip or busybody.
It can also mean a coarse or shallow woman, or a malicious rumormonger.
As you may have noticed, this is basically the same as a yenta. Yachna may refer to a younger person, but it need not. Yenta might also be used in a half-way friendly sense, while yachna would not be.
Yachna was, in the past, a perfectly acceptable name for a girl; it was the female version of the name Johanon or Jachin. At some point, for unknown reasons, it changed meanings.
It is pronounced YOKH-neh, where the KH is a guttural sound (as you might say in 'Bach'). If you are not up to the guttural, just say YOK-neh. It can also be spelled Yachne. See also: yenta and yideneh.