Comes from the term masturbation or wank. A poshey is known to man as a wank whilst wearing a condom, This is can also known as a posh wank. It is believed to be used my blokes who are sensitive, dont like cleaning up after themselves, and those who just preferbeing hygenic and safe. Or even those or a different orientation. If you intend to have a wank with a condom on make sure it is of a good quality and correctly applied, reading the instructions is always advised; otherise it may just split.
Having a wank with a condom on = poshey (posh wanker)