腰 (yāo) is the Chinese character for "kidney" (though snowynight notes in comments that it may only be used to mean this in a food context, not in general). It can also mean "waist" or "middle part". It appears on menus both to indicate actual kidneys and as part of the word 腰果 (yāo guǒ/cashew nut).
When used to mean kidneys, you may see it as 豬腰 (zhū yāo), which is explicitly a pig's kidney, or perhaps as 腰花 (yāo huā), literally "kidney flowers". In the latter case, it means that the kidneys have been cross-hatched before cooking so they open up like flowers when cooked, a similar treatment to that often given to squid.
Note that the radical of 腰 is 肉 (in its ⺼ form in this case). This is pretty common for offal; 肝 (gān/liver), 腸 (cháng/intestine), 肚 (dǔ/tripe), and 肺 (fèi/lung) all share the same radical.
Here are some dishes with 腰 in the name:
|火爆腰花||huǒ bào yāo huā||fire-exploded [quick-fried] "kidney flowers"|
|韮菜炒腰花||jiǔ cài chǎo yāo huā||"kidney flowers" stirfried with Chinese chives|
|蒜茸腰片||suàn róng yāo piàn||sliced kidneys with mashed garlic|
|腰果西芹||yāo guǒ xī qín||celery with cashew nuts|
|腰果叫花雞||yāo guǒ jiào huā jī||"beggar's chicken" (clay-baked chicken) with cashew nuts|
|腰:||yāo||radical 130 (肉)||Cantodict||MandarinTools||YellowBridge||Zhongwen|