Pasatiempo literally translates to "pastime" or "hobby." It's an interesting word because it implies something casual and not serious.
In the context of this song, the singer uses it to express his hurt and anger, stating that he was merely a "pasatiempo en tu cama" (a pastime in your bed) for the other person. This reveals a deeper emotional meaning, highlighting the feeling of being used and discarded, rather than just a simple hobby.
Claudio Alcaraz's "Un Pasatiempo" is a fiery anthem of moving on, where the singer makes it crystal clear that he's over an ex who's trying to come back. He's not holding back, asking, "No sé qué demonios pretendes llamando" [I don't know what the hell you think you're doing calling]. He's feeling great now, and he's not about to let anyone mess that up.
He's pretty blunt about how things have changed, saying, "mis sentimientos me has cambiado" [you have changed my feelings for you] and "ya no me mueve" [it doesn't move me anymore]. The singer is genuinely confused why his ex is even bothering to call, especially after leaving him. He sums it up perfectly when he says, "si sólo un pasatiempo en tu cama" [if I was just a pastime in your bed], emphasizing that he was never more than a casual fling to them. Now, his world is better without them, and he's not looking back!