偉(えら)そうself-important, important-looking, proud

先輩(せんぱい)：「おい、先輩(せんぱい)に偉(えら)そうな顔(かお)をするな！」

Senior: 'Hey, don't give your senior that arrogant look!'

同僚(どうりょう)に偉(えら)そうな態度(たいど)をとる社員(しゃいん)は嫌(きら)われています。

Employees who act snobbish towards their colleagues are disliked.

上司(じょうし)に偉(えら)そうな口(くち)をきいたら上司(じょうし)から叱(しか)られてしまった。

I was scolded by my boss after I spoke to him in an arrogant manner.

兄(あに)が弟(おとうと)に：「おい、兄(あに)に向(む)かって偉(えら)そうな事(こと)を言(い)うなってば！殴(なぐ)るぞ！」

Older brother to younger brother: 'Hey, stop saying such arrogant to your older brother of all people! I'll punch you!'

偉(えら)そうにふんぞりかえっている上司(じょうし)が憎(にく)いのなんのって…。顔(かお)を見(み)るだけで吐(は)き気(け)がする！

I absolutely loathe my boss who's always lounging around acting all self-important... Just seeing his face makes me feel nauseous!