Unconcernedly; nonchalantly — used as an onomatopoeic/mimetic adverb to describe someone appearing or acting without care or embarrassment.
遅刻したのに、彼はのこのこと現れて謝りもしなかった。
Even though he was late, he showed up nonchalantly and didn't even apologize.
泥だらけなのに、のこのこと家に入ってきた。
Covered in mud, he casually came into the house.