Saanwra is another form of the word saanwla, which means one with some darkness in complexion. Saanwre is generally the way to call that 'one', though not in real life. :)
Saanwra or saanwre is mostly used in songs/ poems because Lord Krishna is supposed to have a dark complexion and hence people call him saanwre. Since he's famous for being loved by all, saanwre has more or less become a way to call a male loved one.