Nonplus is a verb that means to perplex or put at a loss of words. It is also occasionally used as a noun that refers to a state of perplexity.
The Latin phrase non plus (known PLUS) means ‘no more.’ Once imported into English, the word became a synonym of ‘puzzle’ and ‘confuse.’
Joel’s odd behavior always seems to nonplus me. I’m most confused by his tendency to dress in swimming trunks when he goes skiing.