Well, it's derived from the word cum, I assume.
I've heard it from some cringy comment which was talking about daddy's cummies for his good little girl or something and the term stuck to my mind over how silly it is.
I like to use silly terms for sexual parts / acts.
You might actually recognize my posts thanks to this.
Like I like to call penis beenus weenus, breasts tiddy and shit like that.
Don't sue me, it's my thing.