Mamey is a delicious fruit that you can purchase on the streets of Mexican towns (in the right season). It's similar to an orange, sweet and over-sized avocado. It is frequently used to prepare licuados at street stands in DF. Mamey is used in two contexts. First, it is used to refer to people with big muscles, especially of the body-builder type, as in "mi cuñado está re mamey" (my brother-in-law is quite muscled).
Second, mamey is a colorful way to refer to oral sex. It comes from mamada (which in turn derives from mamar, literally 'to suck'). Do not confound the meaning of mamey with the other meaning of mamada, as in jalada.
Other popular terms for oral sex are guagüis (WA-weez) and chivo (chee-VOH, lit. male goat). Wichever term you prefer, it is better to precede it with pegar (lit. to hit), as in: "ando medio mareado porque me acaban de pegar un chivo, pero en 5 minutitos me recupero" (I'm a little dizzy cause I just received oral sex, but I'll be fine in a few minutes).