To underwhelm someone is to fail to impress them — the film, the meal, the new phone was fine, nothing was wrong with it, and yet you walked away feeling nothing much. This is a joke word, and knowing the joke is most of knowing the word. English has "overwhelm", which means to hit someone with far more than they can handle. Somebody in the 1950s noticed that if there is an "over", there ought to be an "under", and made one — even though there is no plain "whelm" in ordinary modern English to sit between them. So the word arrives already smiling at itself. That is why you almost always meet it in these two shapes: "I was underwhelmed" and "it was underwhelming". The active form ("the concert underwhelmed us") exists but sounds like a review in a newspaper. The feeling it names is very precise and rather useful: not anger, not disappointment exactly, but a small flat let-down after you had been told to expect something big. That last part matters — you can only be underwhelmed by something that promised. Nobody is underwhelmed by an ordinary Tuesday. Because the humour is baked in, it is not a rude word; it is a polite, slightly dry way of saying "not great" without saying "bad", which makes it perfect for online reviews, and that is where young people meet it most.

沒什麼感覺、不怎麼樣、雷雷的。原本就是拿 overwhelm（壓倒性）開玩笑造出來的字，所以講的是「期待很高，結果普普」的那種平淡失望，不是生氣也不是很難過。台灣講「還好」「沒有到很驚豔」「有點雷」大致就是這個感覺，常用被動：I was underwhelmed。