Well, in Buffy fandom terms, a Duck is a crazy Bangel shipper who, despite Buffy sleeping with several other people, including a vampire who acquires a soul to prove his love, despite moving on emotionally, despite moving on to to different cities, occupations and networks, STILL avow the sanctity of their ship. The name 'duck' comes from this particular fan, who was quite notorious back in the day (and - I realise - still calls herself 'Ducks'). Anyway, Bangel shippers got their name from her. (Of course there are sane and rational B/A shippers, but they're quiet and sensible and don't tend to create drama, and so people don't notice them much.)