へらへらfoolishly (laughing), thoughtlessly (smiling), indiscreetly (talking), carelessly (speaking), frivolously, flippantly, thin (paper, cloth, etc.), weak-bodied, flaring up, bursting into flames

へらへらとした態度(たいど)の社員(しゃいん)を首(くび)にしました。

We fired an employee for their frivolous attitude. {foolishly (laughing), thoughtlessly (smiling), indiscreetly (talking), carelessly (speaking), frivolously, flippantly, thin (paper, cloth, etc.), weak-bodied, flaring up, bursting into flames}

先生(せんせい)：「おい！怒(おこ)られている時(とき)にへらへらするな！」

Teacher: "Hey! Don't smile foolishly when you're being scolded!" {foolishly (laughing), thoughtlessly (smiling), indiscreetly (talking), carelessly (speaking), frivolously, flippantly, thin (paper, cloth, etc.), weak-bodied, flaring up, bursting into flames}

このへらへらの下敷(したじ)きは直(す)ぐに折(お)れてしまうに違(ちが)いない。

This flimsy placemat will surely break immediately. {foolishly (laughing), thoughtlessly (smiling), indiscreetly (talking), carelessly (speaking), frivolously, flippantly, thin (paper, cloth, etc.), weak-bodied, flaring up, bursting into flames}

浮気(うわき)をした事(こと)がバレたのにへらへらしている妻(つま)に苛(いら)ついています。

I'm irritated with my wife who is thoughtlessly laughing even though she got caught cheating. {foolishly (laughing), thoughtlessly (smiling), indiscreetly (talking), carelessly (speaking), frivolously, flippantly, thin (paper, cloth, etc.), weak-bodied, flaring up, bursting into flames}

注意(ちゅうい)されている時(とき)にへらへらと笑(わら)って誤魔化(ごまか)すなんて、失礼(しつれい)極(きわ)まりない行為(こうい)だ…。

Laughing foolishly and trying to deceive when being admonished is an extremely disrespectful behavior... {foolishly (laughing), thoughtlessly (smiling), indiscreetly (talking), carelessly (speaking), frivolously, flippantly, thin (paper, cloth, etc.), weak-bodied, flaring up, bursting into flames}