Yes, knock is playable
...and is worth 17 points.
Found in the following dictionaries
Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)
"Knock" Definition
noun
A pounding or clanking noise made by an engine, often as a result of faulty fuel combustion. Also called ping1.
An instance of striking or colliding; a blow.
Slang A cutting, often petty criticism.
The sound of a sharp tap on a hard surface; a rap.
phrasal-verb
Around Informal To be rough or brutal with; maltreat.
Around Informal To discuss or consider: met to knock around some ideas.
Around Informal To wander from place to place: knocking around Europe.
Knock back Informal To gulp (an alcoholic drink).
Knock down Informal To reduce, as in price: knocked each radio down 20 percent.
idiom
Have it knocked Slang To be certain of success: "He knew he had it knocked after he saw a rough cut of Chinatown” ( Time).
Knock (oneself) out To make a great effort; exhaust oneself.
Knock cold To render unconscious; knock out.
Knock dead Slang To affect strongly and positively: a performance that knocked the audience dead.
Knock dead To kill with a blow.
verb-transitive
To affect in a specified way by striking hard: knocked the mugger senseless.
To cause to collide: I knocked my head on a low beam.
To instill with or as if with blows: We tried to knock some sense into his head.
To produce by hitting or striking: knocked a hole in the wall.
To strike with a hard blow.
verb-intransitive
To collide with something: knocked into the table.
To make a pounding or clanking noise: The car engine is knocking.
To strike a sharp audible blow or series of blows, as on a door.
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