A calzón quitado literally translates to "with underpants removed." It's a common and colorful Spanish idiom that means to speak with complete and brutal honesty, without hiding anything.
In the song, Shakira expresses her frustration and wants to stop beating around the bush. By saying, "mejor hablar a calzón quitado" (better to speak frankly), she's demanding a direct, unfiltered conversation to find out the real truth about their relationship.
Have you ever had a friend or a lover just disappear on you? That's exactly what Shakira is singing about in "Tiempo Sin Verte." She opens by saying, "Llevo tiempo sin verte y sigo aquí / Y no pasa un día que no piense en ti" (I haven't seen you in a while and I'm still here / And not a day goes by that I don't think of you). She's stuck waiting, wondering if the other person even remembers her or the good times they had, like when they were "riendo juntos sobre las olas" (laughing together on the waves).
Shakira isn't just sad, though. She's frustrated and wants answers. She calls out the classic excuse, "Y tu excusa es que andas demasiado ocupado" (And your excuse is that you're too busy), and demands they speak frankly, using the fantastic Colombian phrase "hablar a calzón quitado" (literally, "to talk with your pants off"). She wonders if she did something wrong or was too demanding, but ultimately, she just wants to know if there's any hope for them to go back to how things were, to be "surfearemos juntos como ayer" (surfing together like yesterday).