Foolish
Silly, Folly, Foolishness
noun, な adjective
The kanji and the word are exactly the same. That means they share meanings as well. Since this word is both a noun and a na-adjective, it can also mean things like folly and foolishness.
ぐ
This has the same reading as the kanji you learned, meaning you know the reading!
先ほどは愚なことを申し上げてしまいまして、誠に申し訳ございませんでした。
I am very sorry for the foolish things I said to you earlier.
親友と、愚にもつかない話をして夜を過ごした。
I spent the evening with my best friend talking about silly things.
まさに愚の骨頂だね。
This is certainly the height of folly.