Why is there a final “ein” in “Ich schlaf mit einem Messer ein”?

Question

There’s a Rammstein song which contains the following:

Ich schlaf mit einem Messer ein

I am having a hard time understanding why there’s the last ein?

Could someone be nice enough to tell me if the ein could be removed and it would still be grammatically correct?

Answer 1

German has a feature, that only a very small number of languages have (English doesn't have it): Separable verbs. »Einschlafen« (to fall asleep) is such a separable verb. It is derived from »schlafen« (to sleep), and literally is to in-sleep.

Separable verbs will be split into its parts in many grammatical situations. If this happens, the former prefix floats to the end of the sentence. (Consult your grammar book for more details.)

I compare here »schlafen« (to sleep) and »einschlafen« (to fall asleep):

Ich werde schlafen. Ich werde einschlafen. Ich werde mit dem Messer einschlafen.
I will sleep. I will fall asleep. I will fall asleep with the knife.

Ich schlafe. Ich schlafe ein. Ich schlafe mit dem Messer ein.
I sleep. I fall asleep. I fall asleep with the knife.

Ich habe geschlafen. Ich bin eingeschlafen. Ich bin mit dem Messer eingeschlafen.
I was sleeping. I was falling asleep. I was falling asleep with the knife.

So, to answer your question: Yes, »ein« can be removed, and it would still be grammatically correct. But it would mean something different.

Ich schlafe mit einem Messer ein.
I fall asleep with a knife.

Ich schlafe mit einem Messer.
I sleep with a knife.

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Addendum:
^{(Reaction to other answers who claimed, that »Ich schlafe mit einem Messer« would mean »I have sex with a knife«. This is nonsense.)}

»Mit jemandem schlafen« means: To have sex with somebody. But »mit etwas schlafen« just means: To sleep while lying next to something, or holding something:

Die dreijährige Lisa schläft mit ihrem Teddy.

This doesn't mean, that the little girl has sex with her teddybear. It just means, that she is holding her bear while sleeping, or that the bear is laying in her bed while she sleeps. So

Ich schlafe mit einem Messer.

Does not mean, that you have sex with a knife, because a knife is not a person. It just means: you are sleeping while you have a knife in your bed. (Maybe under the pillow?)

Answer 2

Ich schlaf mit einem Messer [unter dem Kissen] ein

is also correct without »ein«, but sounds odd because of its analogy to

Ich schlaf mit einer Frau
(I'm sleeping with a woman)

Answer 3

The "ein" is the second part of the verb "einschlafen". In German, verbs can be ripped apart, having two pieces, one of which goes to either end of the sentence.

And no, you should not remove "ein" from this sentence, because then it might mean "I'm having sex with a knife", which sounds possibly painful.

But even like it is now, the verb "einschlafen" may mean "to die" (depending on the context), so however you turn it, it stays painful.