Undergoing or suffering the effects of extreme stress: “frequent asides that are often exasperating to today's more stressed-out public” (James Wilcox)
If someone is stressed out, they are very tense and anxious because of difficulties in their lives
Subject to or undergo extreme pressure or strain, as from working. For example, I badly need a vacation; I'm just plain stressed out from this job and its aggravations. The verb stress has meant “afflict with hardship” or “distress” since the 16th century, but the phrase stress out, alluding to psychological stress, dates only from the 1940s