SRS,The reference is to an English trad song.
The song is about a young man on trial for murder.
He goes out hunting and shoots what he thinks is a swan.
The swan turns out to be his lover.
In his defence he claims that she had her white apron on and he mistook
her for a swan.
If you google Polly Vaugh or Polly Von (both spellings are used)you should find the lyrics