to kill a creature because it has an illness or injury that cannot be treated
PUT AN ANIMAL, A BIRD, ETC. OUT OF ITS MISERY
Meaning of PUT AN ANIMAL, A BIRD, ETC. OUT OF ITS MISERY in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005