What word or phrase does your mother always say?
early 15c., inscisif, "slashing, cutting with a sharp edge," from Middle French incisif and directly from Medieval Latin incisivus, from Latin incis-, past participle stem of incidere (see incision). Originally literal; figurative sense of "mentally acute" first recorded 1850 as a borrowing from French. Related: Incisively; incisiveness.
incisive in·ci·sive (ĭn-sī'sĭv)
Having the power to cut.
Relating to the incisor teeth.