1. Physical, mental, or emotional strain or tension
2. Importance or significance attached to a thing; emphasis
3. Physical pressure, pull, or other force exerted on one thing by another; strain
Moving stresses my dog out. So does packing...and cleaning. So does being left at home, especially when any of the aforementioned activities have happened recently. So between moving, my husband packing and leaving on a trip, and me using the vacuum all afternoon, she's a wreck.
|Poor Heidi* (source)|
The original noun definition in the 14th century was "hardship, adversity, force, pressure." The verb version of this word appears around the same time as the noun with the definition "to subject (someone) to force or compulsion." To "put emphasis on" is first attested in 1896 and the psychological sense of being stressed out is attested from 1942.
*I know that's a picture of a cat, but all the dog pictures I found labeled "stress" or "stressed out" were of dogs sleeping or otherwise relaxing in ways only dogs can...