The elastic band on my underwear is all stretched out, so it feels like they are going to fall down.
この場合、次のような言い方ができますよ。
ーThe elastic band on my underwear is all stretched out, so it feels like they are going to fall down.
「パンツのゴムが伸びきってずり落ちそうだ」
the elastic band on my underwear で「下着のパンツのゴム」
to be all stretched out 「伸びきる」
to fall down で「落ちる・ずり落ちる」
ご参考まで！
The elastic in my underwear is stretched out, so they feel like they're going to slide down.
ご質問ありがとうございます。
・「The elastic in my underwear is stretched out, so they feel like they're going to slide down.」
（意味) 下着のパンツのゴムが伸びてずり落ちそうになる。
<例文>The elastic in my underwear is stretched out, so they feel like they're going to slide down. I need new ones.
<訳>下着のパンツのゴムが伸びてずり落ちそうになる。新しいのが必要です。
参考になれば幸いです。
カナダ 
アメリカ合衆国 
