The phrase "put out to pasture" refers to the act of retiring or removing someone from active duty or responsibility, often due to age or obsolescence. Some synonyms for this expression include "retire," "discharge," "dismiss," or "release." These terms all suggest a similar sense of being removed from active service or responsibility. Other similar phrases might include "hang up one's hat," "throw in the towel," or "call it a day." Ultimately, all of these expressions serve to indicate a transition from being actively involved in a particular role or function to being more passive or inactive.