In the 1920s and 30s, the term was occasionally being used in psychological circles to refer to a mental strain or unwelcome happening, and by advocates of holistic medicine to refer to a harmful environmental agent that could cause illness. Walter Cannon used it in 1934 to refer to external factors that disrupted what he called "homeostasis".
Its psychological uses are frequently metaphorical rather than literal, used as a catch-all for perceived difficulties in life. It also became a euphemism, a way of referring to problems and eliciting sympathy without being explicitly confessional, just "stressed out". It covers a huge range of phenomena from mild irritation to the kind of severe problems that might result in a real breakdown of health. In popular usage almost any event or situation between these extremes could be described as stressful. The most extreme events and reactions may elicit the diagnosis of Posttraumatic stress disorder.
