The title refers to a sexual act. In the chorus, you will notice that he says at first, "Finger deep within the borderline," then on the second, "Knuckle deep inside the borderline," and on the third, "Elbow deep inside the borderline... Shoulder deep within the borderline." Clearly, he's talking about putting his arm in something. He may be using this act of constant sexuality as a metaphor - possibly looking at how people will find something they like and then do it over and over again and eventually go on farther and farther until they can't turn back. Depending on your perspective of this song, it may or may not have sexual meaning.
The song deals with how modern technology tends to desensitize people and keeps them from really feeling. It states that only way to really feel true emotions is by experiencing things with other people; really connecting and stripping them of all their outer layers and really penetrating the person underneath that, hence the fisting metaphor. It also states how addictive and primal this is.
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