There are so many interpretations to this song.  At first I was thinking of adding lime to some alcoholic drink.  However, all my drinking friends say they never heard of it.  You have to ask the song writers.  Here's one interpretation from someone else,

"When you put lime in a coconut, the milk curdles and goes sour, thus making the feelings of morning sickness. I believe this song is about a hooker who keeps having massive amounts of sex, and getting pregnant. The song is repetitive because the woman cannot stop repeating this hedonistic treadmill of behavior."

By the way, if you are trying to learn English from such songs, you are doing the wrong way.  The lyrics are trash.  Go and compare those like, "A Stranger From Paradise" and you learn how beautiful the English language can be.

I didn't understand till the end of sense of this song.If someone knows this song help me,please.

Harry Nilson – Put The Lime in the Coconut