put someone out of his or her misery
Kill a wounded or suffering animal or person, as in When a horse breaks a leg, there is nothing to do but put it out of its misery. [Late 1700s]
End someone's feeling of suspense, as in Tell them who won the tournament; put them out of their misery. [c. 1920] Both usages employ put out of in the sense of "extricate" or "free from."
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.