What is....

Have you googled it?

 

You may find this helpful.

https://www.diabetes.co.uk/brittle-diabetes.html

 

Diabetes that looks at the 'rule book' and say 'um no thank you' basically.

For every comment on this forum that says 'do x and y will happen' brittle diabetes means it will not.

It is leaving your door and never knowing what your diabetes will do. It is fear and panic and despair.

And if you do not feel those things you are lucky indeed.

 

The link given by @Guzzler sums up brittle diabetes nicely, at least in my experience as a diabetic over the past 52 years.
The term has been used for years. I recall hearing it when I was attending hospital diabetes clinics back in the 1970 and 80s.
Of course back then insulins were less reliable in absorption and had profiles which, in my personal experience, were less suited to one's diet and lifestyle than those of today. Also there were no insulin pumps, pens or blood glucose monitoring. And the appreciation that rates of insulin absorption differed between different body injection sites was in its infancy.
The worst thing was that, at times, I observed that the term 'brittle diabetes' was used by some doctors as a way of blaming the patient for the less than perfect control of their diabetes.
This 'patient-blaming' behaviour still persists today in various forms not restricted to diabetics alone, although for diabetics the advent and use of insulin pumps has likely helped reduce usage and application of the term.
I copped the 'brittle' title during a time of turbulence to do, I think, with the growth spurts of adolescence when other hormones were playing havoc with my BSLs and I was also under the stress of approaching exams. I was discarded from the 'very good control' class and left to my own devices. I managed without glucose monitors and on the 'not so good, old' Soluble and Isophane insulins to increase doses up to x3, use exercise, get through exams and thus 'survive' the "brittle diabetes appellation ride".
What to do after that? Change doctor and for myself, be careful of using labels for others.