The Chinese word for bottom is
臀

tún

Chinese Definition

bottom
1. 底壁

底部
1. n. bottom (lowest part of something)

底端
1. n. bottom part; bottom

底
1. bottom; underneath; underside
2. ground; background
3. ins and outs; actual situation
4. master copy
5. end (of a year or month)
六月底 - end of June -
6. remnants; remains; leftovers
7. (Min Nan) inside
糜底 - in the congee, MN-T -
8. (mathematics) base (of triangle, logarithm, etc.)
9. (Cantonese) staple food as the base of a meal
伊麵底 - e-fu noodles as the base (e.g., for lobster) -
加底 - to add rice in a meal -
10. (Hong Kong Cantonese) tendency in one's personality; leaning
燥底 - having a bad temper -
紅底 - having pro-CCP leanings -
Mem1底 - having a masochistic side -
11. (Hong Kong Cantonese) criminal record
佢個底花嗮 - His criminal record is all dirty and messy.
12. (Cantonese) underpants
紅底 - red underpants -
露lou6底 - to expose one's underpants -
13. Short for 底細
14. (Min) which; what