The process of, while penetrating a partner from behind (Either anally or vaginally), reaching around and playing with his/her genitalia.
Contrary to popular belief, not only a homosexual act.

Lying flat refers to a defeatist lifestyle in Chin, where people stop working, desiring material acquisition, and tap out of any social life – sometimes for good. By lying flat, young people are participating in a kind of non-cooperation movement, rather than surrendering themselves to China's oppressive work culture where they see little hope of social mobility.

1. n: the performance of the act of masterbating a sexual partner, usually while the person performing this act is simultaneously performing doggy style, anal sex, or spooning on the same partner as the person on top.

See also: hand job, jacking off

2. n: a metaphor of def 1 describing compensation, usually derogetory.

An alcoholic beverage made from Jägermeister and Schweppes Agrum.

when two men are having gay sex and the man behind reaches around and jacks off the guy in the front

When your partner gives you a handjob from behind.

A reach-around is when 2 men are having anal sex and the one at the back reaches round and gives the man being bummed a hand job as he is receiving no pleasure from the sodomy