What you want to use is a transparency mask! You can do this directly in Blender, or use an image editor, if it's easier. First unwrap your panties, then go to your UV/Image editor and create and save the file at the resolution you want. Remember that file name; it should have appeared automatically in the materials (check all of them if you have more than one), but if it doesn't, you can simply import it manually.
You may have to adjust the setup, anyway. Switch back to wherever you have your node editor open. Once you have your Image Texture with that file you saved (it should be black, but that's actually perfect for this), add a Texture Coordinate node (you'll find it under 'input' in the 'add' menu). Hook the UV slot to the Image Texture's vector slot. From this point onwards, you have a couple of options, as far as setup is concerned. If you're already using a principled shader node, simply plug that right into the transmission slot. (You may have to make it wider to see all the words, so you can find the right one.) If you already have a completely different setup, no worries! Add a transparent shader and a mix shader. Plug your original setup to the top slot, and the transparent shader to the bottom slot. Now hook up your Image Texture to the mix slot.
If you've done everything right, at this point, your panties should still look the same. If they've vanished, or look partly transparent, you've done something wrong (don't fret, if you did, it's easy to do; try zooming in with the mouse wheel and connecting your nodes, and then rearranging them after). Once you have that sorted, make sure you have your Image Texture selected, then go back to where you've been working on your model. Go to material view and zoom in to where you want the panties to be see-through. It's time to switch to texture paint! Make sure you use only a black-and-white brush for this. It can be gray. The idea is all the parts you paint white will be completely transparent, and the parts that stay black will be completely opaque. Gray will mix between the two. To this end, I recommend using a brush with a soft edge to get that gradual affect you're after. As you're painting, you should see it turning transparent under your brush. If you add too much, you can soften or remove it by using gray or black instead of white. Make sure to save your image in the UV/Image Editor before you close Blender. Blender won't do it for you, even if you save the .blend file! You can always pick back up where you left off, if you're not finished, yet.
I hope this helps! Happy Blending!