It has a few meanings: scruple, dread, hesitation, restrain or 顾忌

Example sentences:

她破口大骂，毫无忌惮。

She shouted abuse without any restraint.

他毫无忌惮地发表了他的意见。

He had the courage to express his opinion.

他毫无忌惮的贪求名利。

He has uncontrollable greed for fame and wealth.

I hope it helps and wish you good luck in learning Chinese!

经常在成语 肆无忌惮 ( sì wú jì dàn ) 中出现。这个成语的意思是为对事情没有畏惧， 没有顾虑。跟“胆大包天”的感觉差不多。

 例如：小明真的是肆无忌惮，连老师都不放在眼里！

Translation: Ming is not afraid of anything(negative meaning), he is even not afraid of the teachers!( The sentence literally means " Ming does not put the teachers in the eyes", meaning that he is looking down on the teachers.)