set in
(of rain, bad weather, infection, etc) begin and seem likely to continue (指雨、 坏天气、 传染等)开始并可能继续下去
I must get those bulbs planted before the cold weather sets in. 我得在天气转冷之前把这些球茎秧种上.
Those beams will need to be replaced; it looks as though woodworm has set in. 那些大梁该换了, 看样子里面生了蛀虫了.