The verb is "to know." To know by heart" is a separable phrasal verb meaning to know something very well. "Heart" is a noun. The prepositional phrase "by heart" completes the sense of the phrasal verb (as an adverbial).
February 1, 2017
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"to know something by heart" is to have it memorized.
February 1, 2017
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There is no verb in Standard English for "heart". In informal or slang, it means "to love". E.g. I heart New York. I suggest you master standard English first before delving wholeheartedly into slang.
February 1, 2017
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