扒灰

páhuī

(1) [an affair between father and daughter-in-law] 公公与儿媳通奸

俗呼麀为扒灰。 --明·冯梦祯《快雪堂漫录·书王文旦事》

(2) 又清李元复谓爬行灰上则膝污, "膝"与"媳"偕音, 故"扒灰"即"污媳"之隐语