means An automatic, involuntary response to a stimulus that happens without conscious thought, like your knee kicking when tapped or your hand jerking back from a hot stove.

from From the Latin 'reflexus,' the past participle of 'reflectere,' meaning 'to bend back' — from 're-' (back) plus 'flectere' (to bend), the same root that gives us 'reflect.' Originally the word described light or heat being thrown back, and only later did physiologists borrow it for the way a nerve signal 'bends back' from a stimulus into an action, without ever troubling the brain for permission.