This is the 3083rd most frequent Chinese word.
酸
Sour.
'酸' here describes the taste and indicates that it is tangy or tart.
这些柠檬好酸啊！
These lemons are so sour!
'酸' here describes a feeling in the body, indicating soreness or aching muscles.
最近工作太多，感觉肩膀有点酸。
Recently, I've been working too much and my shoulders feel a bit sore.
'酸' here is metaphorical, describing an emotional feeling such as jealousy or bitterness.