What does "不可描述" mean?

词性：动词 词义：执行交配 来源：楚天都市网新闻 - 23日凌晨，湖北宜昌高警在一隧道内发现停着一辆小车，向车内一看，竟有对男女在“不可描述”。民警敲窗3分钟，两人才提着裤子慌张接受检询，理由竟是“精神不好，提提神！”后民警对其进行了安全教育后，两人迅速开车驶离。

ENPart of speech: Verb. Meaning: to have sex. Source: Chutian Metropolis Daily News - In the early morning of the 23rd, the Yichang traffic police in Hubei found a parked car in a tunnel. Looking inside, they saw a man and a woman doing something 'indescribable'. The police knocked on the window for 3 minutes before the two of them pulled up their pants and nervously accepted the inquiry. The reason was 'not feeling well, just wanted to perk up!' After the police gave them a safety education, the two quickly drove away.