We arrived at the mountain at seven o'clock in the morning, and because it had been raining the previous night, the rocks were slippery.

slippery 是形容词，由动词 slip（滑倒）加上后缀 -ery 构成，表示"容易使人滑倒的"或"表面光滑的"。这个词常用于描述因潮湿、油污等原因导致表面摩擦力减小的状态。