In Puccini's most famous opera, "Tosca", Baron Scarpia blackmails the popular singer, Floria Tosca, into sleeping with him in exchange for her lover Cavaradossi's life. 

When he triumphantly comes to embrace her however, she thrusts a knife into him, exclaiming "Questo è il bacio di Tosca!" "This is Tosca's kiss!"

So I guess you could say it's an euphemism for not getting quite what you expected.