Thoughtlessly
Unreasonably, Recklessly, Absurdly
adverb
"Darkness is nothing to worry about," I said thoughtlessly and unreasonably. Of course darkness is worth worrying about. It's darkness! Who knows what is in there!
The readings are the ones you learned with the kanji.
無闇に女の子に声をかけるのはよした方がいいよ。
You shouldn’t hit on girls indiscriminately.
海外では夜、無闇に外を出歩くと、危ない目にあうかもしれないよ。
Thoughtlessly walking around at night in a foreign country might put you in danger.
無闇に「みだらな」という言葉を使わない方がいいよ。
You shouldn't use the word "lewd" thoughtlessly.