The phrase 'rather frail' is correct and can be used in written English.
You can use it when you want to describe someone or something that appears weak or delicate. For example: The old man seemed rather frail and had trouble walking.
"I was rather frail for a while".
He was elderly and rather frail, but had an air of ruling-class authority.
Naipaul's wife, Pat, was a pretty petite woman in her 30s who seemed rather frail.
But beyond the fancier bits of London, the national market still looks rather frail.
She is actually rather frail, though that is well disguised by her erect posture and almost unlined complexion.
Father Wilhelm Kleinsorge, of the Society of Jesus, was, on the morning of the explosion, in rather frail condition.
He was perhaps a few years older than she was, tall and rather frail-looking, slightly hollow-chested, but vigorous in his greeting, relentless in his smiling.
Dr. Peltzer, the great German runner, slim, blond, rather frail-looking (he was not competing) watched earnestly and in some surprise, his rival, Hahn, who is a stocky and powerful Nebraskian, finish his record half mile apparently strong.
Being pre-frail (rather than frail or non-frail) was not significantly associated with poor caregiver status, suggesting it is not until an individual accumulates roughly 12 13 deficits (FI score ≈ 0.3) that caregivers feel that they are unable to continue to care.
In his book Physique and Character, first published in 1921, he wrote that among his patients a frail, rather weak (asthenic) body build as well as a muscular (athletic) physique were frequently characteristic of schizophrenic patients, while a short, rotund (pyknic) build was often found among manic-depressive patients.
The unfurling ribbons of, usually, primary colors are delicate, rather than frail, and shift emphasis to the fiercely white grounds that they cross or incise.
