A Walrus Called Shagbat

[Skylax]

This topic is not related with Greek or Latin, but, as this board is open...

During WWII, the British FAA used the Supermarine Walrus three/four seats single-engined biplane amphibian aircraft (How do you say that with a single German word?) for SAR missions. This aircraft was dubbed a "Shagbat" by his crews.

My question is : what is a shagbat ? Following a book about British military aircraft, a shagbat was "a legendary bird, whose reputed ever-decreasing circular flightpad had an inevitable conclusion". Does someone know more about this bird and legends? Did some Granny tell stories about it?

[Keesa]

Mine didn't. (I wish she had.) But it sounds to me like some kind of mythical bird who could only fly in a downward spiral, although I don't see what the connection would have been between the bird and the aircraft. Maybe the crews just liked the sound of it?