what doe slishity slosh mean in the poem rain by shel silver stein?

I opened my eyes

And looked up at the rain,

And it dripped in my head

And flowed into my brain,

And all that I hear as I lie in my bed

Is the slishity-slosh of the rain in my head.

I step very softly,

I walk very slow,

I can't do a handstand--

I might overflow,

So pardon the wild crazy thing I just said--

I'm just not the same since there's rain in my head.