As you can see here, it appears that Mervyn Peake invented the word.
Here is another theory: On the other hand, I believe that rabous (as in a bird taking off ‘from that rabous landing ground’) is probably an accident, a mis-transcription (that went unnoticed) of Peake’s manuscript by his typist. The MS of this passage no longer exists, but knowing the idiosyncracies of Peake’s handwriting, I feel sure that he wrote ‘rufous’, a reddish-brown colour, perfectly applicable to the Countess’s hair in which the bird had been perched.
Taken from Mervyn [Laurence] Peake: FAQ
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