plumbing question/advice needed

[Xiaxio]

So we have had issues with this plumbing for years and I'm tired of it, so I finally decided to give it a go considering im the only man in the house and wanting to helping out :lol:

So here's my question:
Does anybody have an idea what this part is called? It's the nut/ring looking object that connects to the drain under the sink. Sorry for sounding like an idiot. Gotta learn somehow I suppose lol
it seems to only be leaking from the top, so it shouldn't be any other problem other than the fact that it does not want to stay connected when twisting onto the threads.

(sorry for bad quality

 

[obewan]

If it won't tighten up it sounds like the threads in the nut have stripped.
A new one is the answer or you may be able to get it to 'bite' again using some plumbing sealing tape.
if it's only leaking slightly you may be lucky.

Rubber washer at the top doesn't look in good condition either

 

[john-e89]

In-do it from the elbow at the other end and take the pipe and washers to a plumbers shop, you need 2 new nuts and 'o' rings and sealing washers, the green rings. In the UK we have 40mm waste and it looks the same there in the US but obviously I can't be sure on size, the shop will know. You could try to seal it up as said but for the cost of the bits, a few dollars worth, you may as well get all new seals and nuts.

When you re-fit the pipe BE GENTLE, plastic threads strip very easily, take your time to 'feel' that the nut is going on square, if it starts to get tight after only a turn or two stop and un-do it or you'll strip it again, it will go on nicely and screw up for approx 6-8 turns before tightening if you've got it square.

Best of luck.