| Nothing directly to do with cigarettes or homosexuality, this is the phenomenon whereby someone new to a conversation will be misled as to the true subject of discussion, and will venture off-topic or otherwise useless or irrelevant information, usually as part of an attempt to participate in the conversation. This, in turn, is likely to be part of a grander scheme to attempt to get a life. People who already have a life usually don't make the fag-end mistake on account of being more socially experienced.
This phrase is used predominantly in the UK. In the US, I hear that it is otherwise known as picking up old stogies, dipping without a chip (thanks Tato) and (at a stretch, and not quite the whole meaning) 'poking one's nose in'. | 
