In aesthetics and taste, shibu is extended to an adjectival form, shibui, which indicates "astringent" and/or "sour," and a noun form, shibumi, which denotes a profound spiritual aspect.
When used in the meaning of sour, the connotation of shibui can refer to either flavor or appearance, as in sour, sullen glances, the latter of which is notably reflected in many noh masks and, until recently, was a facial expression common among many elderly men.
The name "shibushi" originated in the Muromachi period (1336-1573) to describe a sour or astringent taste, similar to an unripe persimmon.
Shibui still maintains this literal meaning, and remains the antonym of amai (甘い), meaning "sweet".
Shibusa's base form is shibu, which means "persimmon juice" - or, on a more subtle level, the astringent flavor of persimmon juice... a taste that has a remarkable resonance with the Japanese perspective of aesthetics and the ideal form of human behavior.