I can no longer feel my derriere because I have been sitting on it all day.
I have been sitting so long that my butt hurts.
In these sentence examples, I have provided you with three different terms for the word bottom. If you want to be very informal and frank, you may use the term "ass", but I do not suggest you use this in everyday language unless it is with your friends. It can be very classless and unprofessional in formal or business settings.
Ass/arse/bum/rear end - these are all slang words for someone's 'bottom' or 'posterior.'
To go numb - we say this when a part of your body has the blood supply restricted resulting in loss of sensation to that part of the body.
"I've been working on the PC all day - my arse is really aching!"
"Mine has just gone numb - I can't even feel it!"
Ass/arse/bum/rear end -これらはお尻を表すスラングです。
To go numb - 体の一部の血流が悪くなってしびれて感覚がなくなってしまうことを言います。
"I've been working on the PC all day - my arse is really aching!" 一日中パソコンで作業してたから、お尻が痛い！
"Mine has just gone numb - I can't even feel it!" 僕なんてもうしびれて感覚ないよ。何も感じない。
I have numb bum...its aching from sitting so long!
The word bum is used in everyday language as the common reference to our
posterior/bottom/ arse(slang).
It is a sign of fatique when blood circulation is restricted to the point of pain or ache or the dreaded "NUMB BUM!"
Sitting a long time makes our butt go numb or fall asleep. When that happens most people say "My butt fell asleep" meaning you can't feel it at the moment. There are different ways to describe it. For example butt, bottom, backside,junk in the trunk, behind, and bum. Once you stretch your muscles you should be able to regain the feeling again.
長時間座っていると、お尻の感覚がなくなって麻痺してしまいます。そのときには、"My butt fell asleep"と言う人が多いです。この意味は、一時的に感覚がなくなっているという意味です。その他にも表現する方法はあります。
例えば、butt, bottom, backside, junk in the trunk, behind, bumなどです。筋肉を伸ばしてあげれば、感覚が戻ってきます。
You can refer to your bottom as your 'bum' 'butt' or 'buttocks' in English.
To say that it hurts from sitting down for too long you can tell someone that 'it aches' or 'it hurts' or you feel 'uncomfortble due to sitting down for so long'
If you no longer want to sit down you should say, 'I need to stand, my butt hurts from sitting down for so long'.
Hope this helps
Teacher Jemxii
お尻のことは英語で "bum" や "butt"、"buttocks" と言えます。
「ずっと座っていてお尻が痛い」という時は、「it aches（痛い）」 や 「it hurts（痛い）」、「uncomfortable due to sitting down for so long（ずっと座って居心地が悪い）」と伝えるといいでしょう。
もう座っていたくないのなら「I need to stand, my butt hurts from sitting down for so long.
（立ちたい、座りすぎてお尻が痛い）」と言ってみてください。
参考になれば幸いです。
Teacher Jemxii
There are many ways in which one can refer to one's bottom. You could use "my behind", "bum" or "rear-end" just to name a few examples. We use the word "sore" when something mildly hurts. If your behind has no feeling, we usually use the word "numb". Any of the above sentences should suffice! Hope this helps and thank you for your question.
アメリカ合衆国 
日本 
イギリス 
南アフリカ 
