I'm a new user so I can't post URLs!
I too was puzzled by this spelling… Searching with DuckDuckGo landed me on your message. Putting "Cuíosach" into both my computer's Focalbeo app and into teanglann DOT ie brought up:
cúiseach, a1. Well-mannered; prim, demure.
I felt this wasn't quite what I wanted to express. I wanted to say that I am fair or middling.
So I went here:
focloir DOT ie and put in 'fair'
No 5 on the list is 'moderately good'
I asked it to "Show Full Entry"
5 adjective moderately good
measartha adj3 C M U
cuibheasach adj1 C M U
réasúnta adj3 C M U
they had a fair start to the season chuir siad tús sách maith leis an séasúr
Going back to Teanglann
cuibheasach, a1. 1. Fair, middling. ~ maith, fairly good. Tá siad ~ gan a bheith maíteach, they are fair to middling. 2 = CUIBHIÚIL. (Var:cuibhdheasach)
Then I went to Focalbeo and it has
fair adj., middling adj.