It's just a euphemism for marijuana. "Unda mi sleng teng" is being under the influence of marijuana.
But not all. So quite offtopic, but it reminds me of a related topic, but not as funny or peaceful.
In my youth, this song was popular in the clubs we went
TOK - chi chi man
https://m.youtube.com/watch?v=xCrhfXjIEvY
We loved it. Very energetic. (my body still wants to dance hearing that riddim)
It was much later, that I learned, what "chi chi man" and "bun them" means.
Burn the homosexuals.
To which the artists say, it is only meant in a metaphorical, spiritual fire. Burn the filthy, gay thoughts. But since gay people get murdered and burned for real in jamaica, I cannot really enjoy it anymore.
(but I did enjoy the dark humor a bit, witnessing real homosexuals sing and dance to that song, while still being ignorant of its meaning)