Since it was sampled from Ann Peebles original song, which was about a man she was no longer with and how she hated the rain because it reminded her of memories of him, I figured Missy's song was along the same vein?
However, Missy's song was more about how she was trying to get over him by being "fly" (flexing), smoking, cruising around in her droptop, listening to music, going to the beach to hang w/friends.
But the rain still kept reminding her of their relationship hence the line "Oh Missy, try to maintain.."
You know it was about a relationship because she ends the song by "Chumpy, I break up with him before he dump me, To have me yes you lucky (I can't stand the rain!)