"Wet" and "damp" are two words that can be used to describe something that is moist or humid. "Rainy" can be used to describe weather that is characterized by rain.
"Wet" comes from the Old Engpsh "wæt," which means "moist" or "watery."
For example:
"The ground was wet after the rain." (地面下雨后变得湿湿的)
"He wiped his wet hands on his pants." (他把湿手擦在裤子上)
"Damp" comes from the Middle Engpsh "damp," which means "moist" or "humid."
For example:
"The walls were damp from the humidity." (墙壁因湿气而变得潮湿)
"The damp air made it hard to breathe." (潮湿的空气使人难以呼吸)
"Rainy" comes from the Old Engpsh "regnian," which means "to rain."
For example:
"Today is rainy, I hope you bring an umbrella." (今天下雨了，希望你带把伞)
"The forecast said it will be rainy tomorrow." (天气预报说明天会下雨)