Don't know, it's an insider joke with the production crew. Most of these phrases are as well but that one is unknown. Either way it's probably suppose to be something so rude and vile, your mind can try to figure out a direct translation.
Hey, I've searched a long time to know a bit about Stitch's language, and this is cool that you have found this! But I have Googled some time now, and I've found Stitch's language!!! It's called TANTALOG, just google "tantalog language meaning" and it'll pop up right in font of ya!
But there is no: : Meega nala quista/kweesta = (Some sort of naughty or threatening phrase.) Tookie bah wah = Let's get started (or 'let's begin' Iki bah bah = Come on Naga- no Soka = Sorry Okie-taka = Okey-dokey Gaba = What Achi-baba = Friend