Easton's Bible Dictionary (1897)
From Latin levir, “a husband’s brother,” the name of an ancient custom ordained by Moses, by which, when an Israelite died without issue, his surviving brother was required to marry the widow, so as to continue his brother’s family through the son that might be born of that marriage (Gen. 38:8; Deut. 25:5-10; comp. Ruth 3; 4:10).