Book excerpt: Visceral, raw, and - above all - utterly filthy, The Big Book of Filth is an ideal companion for the lexically incorrect, and the perfect gift for those who worry that their vocabulary of sexual vulgarism is limited to such oft-used terms as cock, knob, tits and wanking. Here are hundreds of words and phrases you didn't know, some you probably didn't want to know - and words and phrases for sexual practices you had no idea ever even existed.