In northern dialect this character means the buttocks; standard Chinese says 屁股.
Tips
history
腚 is a regional (mainly Shandong / northern) word that does not appear in standard written Chinese, where 屁股 is used. It is a body-part char: the flesh radical ⺼ plus the phonetic 定.
register
Dialectal and colloquial — heard in northern speech and seen in dialect writing, not in formal standard Chinese.
定 supplies the sound dìng with no meaning contribution. It is a familiar standalone character meaning to fix or settle, reused here purely for its reading.
