Sitting around in the bath was of course a favourite occupation of Douglas Adams, which is why it's a reoccurring theme in the Hitchhiker's Guide to the Galaxy -- and why one of the main characters wears a dressing gown throughout. This may perhaps have made the phrase popular?
The fourth book in the trilogy is now being brilliantly serialised back where it all started, on Radio 4: 6:30 pm on Tuesdays, and 11 something on I think Thursday nights. (For non UK people, www.bbc.co.uk, Radio 4, Listen Again).
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