Chamaca is a colloquial term, primarily used in some Latin American countries, to refer to a young girl or a kid. It's a more informal and affectionate way of saying "girl" or "young woman."
In this song, the singer, who is 34, refers to his 19-year-old love interest as a chamaca. He then playfully adds that when he's with her, she makes him feel like a chamaco (a young boy), highlighting the youthful energy and joy she brings into his life despite their age difference. This word adds a touch of endearing informality and cultural flavor to the lyrics.
This is a song about going public. The singer is tired of pretending that nothing is going on, because everyone around them has already worked it out. Word has spread through the neighbourhood and around the dance floor, so hiding it has stopped making any sense.
He puts the awkward part on the table himself. She is nineteen and he is thirty four, and he knows exactly how that looks to other people. His answer is that she makes him feel young again, and that he would rather be honest about it than keep sneaking around.
The chorus is the whole argument in one line. If she is the one he wants, why hide? Let people talk, let them call it disrespectful, let them bring a camera and film the two of them together. He is not going anywhere.
Underneath the bravado there is something softer. He calls what they have an addiction to their own sickness, admits he is going crazy waiting to see her, and says the two of them were born to be together. It is a plena track built for a dance floor, but the message is simple enough: stop hiding.