the.com/knock
the universal password to a door that has no idea who you are
means To strike a surface with a sharp blow, often to make noise or signal one's presence, or the sound such a blow produces.
from From Old English 'cnocian,' meaning to strike or pound — an echoic word, born from the very sound it names, the dull thud of fist on wood. It has cousins across the old Germanic tongues (Old Norse 'knoka' among them), all of them basically the mouth imitating the hand.
shave and a haircutthe two-knock rhythm answered with two beats
engine troublepremature fuel ignition is literally called engine knock
superstitionknocking wood to ward off jinxes predates Christianity
morse cousinprisoners tapped grid codes through cell walls to talk
opportunitythe proverb says it knocks exactly once