Muttering under your breath in such a way that you cannot but help make yourself heard.
This first came into common use after the website screamingwhisper.com grew in popularity as a place where adults could voice their opinions without being worried about text speak and other incomprehensible Internet abbreviations.
If someone fails to say thank you the you might quietly say "Don't mention it." ... then under your breath you'd mutter "Oh, sorry you didn't" and then later on you might want to share your frustration with others or even post all about your mood on an internet message board. The culmination of this is known as having a screamingwhisper.