fragile or frail
Frail.
slender
There is one syllable in the word frail
He was very frail, frail meaning weak.
slim or thin
Yes, "frail" does not have a suffix. It is a standalone word.
The word "frail" is an adjective that describes someone or something that is delicate or weak.
other words for weak are feeble, frail, fragile, infirm, decrepit, debilitated.
bleak word frail quail
No, it is not, despite the -LY ending. Spindly is an adjective, meaning slender, elongated, especially indicating a delicate or frail nature.
Towards the end of her life, my grandmother was very frail.