Question

What does “I know that song word for word” mean?

Answer 1

Though it is possible that the person has studied the words in the song word by word it usually means they have played and listened to the song so much that they know it word for word and can sing it without any mistakes.

Step-by-step explanation:

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Answer 2

It means that they are so familiar with the song that they could revise it identically - in simpler words, they can sing it without any mistake.