nonplussed

The word originates from the Latin phrase 'non plus,' meaning 'no more' or 'not further.' This phrase described a state where a person could go no further in thought or argument, signifying a standstill of understanding. In the 16th century, it was adopted into English as a noun ('a nonplus') meaning a state of perplexity. By the late 16th century, the past participle 'nonplussed' emerged as an adjective to describe someone brought to such a state of bafflement. The modern, chiefly North American informal meaning of "unperturbed" is a later development, likely arising from a misinterpretation of the word's form as simply "not bothered."

Structure: non (not) + plussed (perplexed) - non: Latin origin, a negative prefix meaning "not." - plussed: From the past participle of the obsolete verb 'pluse' or 'plus,' meaning to puzzle or perplex, itself from Latin 'plus,' meaning 'more,' via the phrase 'non plus' (literally 'no more').