lala comes from an old expression “ooooohlala” which meant damn your fine or wow I want to sex you. It has been warped by at least two generations to be lala and refers to sex acts or being aroused. It originally came from the French language and like all slang has been changed and adopted by the next generation so they can claim to be new and fresh when in reality they are emulating their parents but refuse to acknowledge it.
father: oooohlala she is drop dead sexy
Son: lala dad?
Father no, oooohlala, lala just menas your goofy in the head