Cosquillitas is the diminutive form of cosquillas, meaning 'tickles'. The addition of '-itas' makes it sound endearing and emphasizes a light, playful sensation.
In the song, Toby Love sings, "Casi siento mil cosquillitas dentro muy dentro" (I almost feel a thousand little tickles deep inside). This word beautifully conveys the nervous, excited, and almost overwhelming sensation of being on the verge of falling in love, making it a memorable and evocative choice.
Toby Love builds this whole bachata around one word: casi, almost. He's talking to a friend, someone he calls "amiga mía," and confessing how close he came to crossing a line with her. "Por poco yo me entrego a ti" (I almost gave myself to you), he admits, and from there he just keeps listing near misses: he almost stole a kiss, almost told her how much he liked her, almost said something he couldn't take back.
The hook keeps circling the same held breath. "Que casi siento mil cosquillitas dentro muy dentro" (I almost felt a thousand little tingles deep inside) is a funny, physical way to describe restraint, like his whole body wanted to react and he talked himself out of it every single time. By the end he's still stuck in the same loop: "Yo casi digo sí, yo casi digo sí" (I almost said yes, I almost said yes). Nothing ever actually happens. That's the whole point of the song: he never crosses over, he just keeps almost doing it.