Prefix used in a few set words like 優しい (やさしい) and 優男 (やさおとこ). Not freely productive; mostly encountered inside established compounds.
The 優 in 優しい is used as a prefix expressing gentleness.
The word 優男 carries the meaning of a man with a gentle impression.
gentle-natured man · man of delicate features; slender man · effeminate man; feeble man
gentle woman; affectionate woman · beautiful graceful woman
graceful figure; elegant appearance
slender figure; slim build
gentle-looking; kind-looking; sweet-looking
The prefix 優 (やさ) is related to the adjective 優しい (やさしい) and carries the core meaning of gentleness or tenderness. Its historical derivation is uncertain, but it appears in a small set of native Japanese words.