Real Dictionary

knock
verb: If someone knocks two rooms or buildings into one, or knocks them together, they make them form one room or building by removing a wall.
verb: If you knock on something such as a door or window, you hit it, usually several times, to attract someone's attention.
verb: If you knock something, you touch or hit it roughly, especially so that it falls or moves.

knock in American English
to hit; strike
to strike a blow or blows with the fist or some hard object; esp., to rap on a door
intransitive verb: to strike a sounding blow with the fist, knuckles, or anything hard, esp. on a door, window, or the like, as in seeking admittance, calling attention, or giving a signal
transitive verb: to give a sounding or forcible blow to; hit; strike; beat

knock in Automotive Engineering
noun: Knock is the noise caused by part of the air-fuel mixture in an engine cylinder burning before the normal combustion started by a spark.

knock in British English
verb: to give a blow or push to; strike