Bathroom sink has been making a bad odor. I though the P trap needed cleaning. When I went to unscrew the plastic wing nut things, I noticed the part between the sink drain and P trap was loose. I am surprised no water had leaked out from it. I measured it and the drain pipe from the sink outer diameter is 1.25" and inside the trap is 1.50". There was a little plastic washer looking thing but it has an extremely large amount of play on the 1.25" pipe. I can have that plastic washer on the 1.25" and stick a screw driver shaft between the washer and pipe, that is how much play there is. Is this just a super worn out washer? Or did the previous owner of my house Jimmy rig this? Do I need to somehow make my sink drain pipe 1.50" or is there a big washer that can accommodate the 1.25" to 1.50"? I don't know what I am looking for at Home Depot or Lowe's so if you guys know what I need it would be greatly appreciated if you tell me so I can go buy this to fix this.

It's my only toilet so I don't wanna start another repair project if you feel me.