Literal translation
to have the seagull's illness
Meaning
to have an imaginary or faked illness
Explanation
To have a fake or imaginary illness, literally the seagull's disease, sick head, healthy beak. The seagull, clumsy on the ground but gluttonous at feeding time, inspires the image of someone who looks unwell yet keeps a hearty appetite, on the idea that as long as one eats, it is a good sign. The rhyme between gabian and san likely helped the expression stick.