Foolish

Foolish is an adjective, and it has the following meanings:
1: lacking in sense, judgment, or discretion
2a : absurd, ridiculous 2b : marked by a loss of composure
3: insignificant, unimportant
Here are a few examples, which you could match with the meanings above:
He was wearing a foolish grin.
She's been taking foolish risks.
Those clothes make her look foolish.
She must feel foolish wearing those clothes.
I never thought you'd be foolish enough to believe him.

It's worth noting, though, that for some people, being foolish is not a bad thing. Steve Jobs famously says at the end of his Stanford commencement address: "Stay hungry, stay foolish." Here he means that people should keep taking risks, even if other people think they're lacking in judgment.
In the Tarot deck, The Fool represents innocence and a sense of adventure, and often signifies the beginning of a journey.
In the song, These Foolish Things, the idea is that insignificant things remind the speaker of her lover. Here's a version sung by Ella Fitzgerald:

I think it's safe to say that acting foolish is commonly associated with being in love, or being very engaged with something, or having a great passion for something. Inventors have often been considered foolish, until their inventions were found to be useful or important. Creative people are sometimes seen as foolish, because others may not understand their obsessions and therefore find them very impractical.
What do you think? Is being foolish only negative, or could it have some positive possibilities?

Finally, the noun of this adjective is foolishness, and the adverb is foolishly.