Of software, used to emphasise that a program is unaware of some important fact about its environment, either because it has been fooled into believing a lie, or because it doesn' t care. The sense of "happy" here is not that of elation, but rather that of blissful ignorance. "The program continues to run, happily unaware that its output is going to /dev/null." [href="module.php?name=Lexikon&file=search&eid=1&query=Jargon File">Jargon File]
