The song was originally written by the Mighty Spoiler, he won the calypso crown with it in 1953 I think.

The mystery word is "partner", Spoiler often used this as a punctuation word in his songs.

The other line is "I want you to believe me, so help me bless"

Spoiler has another verse:

So I went and ask Mr Ross
He say he beg the devil to turn him a horse (hoss)
And I ask another fella they call Lawrence
He say he want to come back as a big black wood-ant
But they too foolish, when you turn a horse
You got to carry people's load, get licks from your boss
And as a wood-ant is old wood you have to eat
But as a bedbug I'll be biting the human meat!

There are many recordings of this song from the 1950s -- Invader recorded it more than once, and I have several versions by Jamaican singers.

Spoiler is a brilliant calypsonian and should be better known -- his other songs include Cat Brain, about his sister and their cat exchanging minds; Talk Backwards ("Suppose every woman could afford/to sit down and learn how to talk backward"); Indian Lady:

So the Indian Lady
Took a pound of flour and make a roti
Then she put a radio ariel in the roti, yes partner
And she hear Indian music from Calcutta.