COOL BLOOD. A phrase sometimes used to signify tranquillity, or calmness; that is, the condition of one who has the calm and undisturbed use of his reason. In cases of homicide, it frequently becomes necessary to. ascertain whether the act of the person killing was done in cool blood or not, in order to ascertain the degree of his guilt. Bac. Ab. Murder, B; Kiel 56 Sid. 177 Lev. 180. Vide Intention; Murder; Manslaughter; Will.