noun, na-adjective, noun or participle which takes 'suru'
laziness; shirking; cutting corners
Describes a person's attitude or behavior of avoiding effort, doing things the easy way, or neglecting proper procedure. Often used with する to mean 'to slack off' or 'to take a shortcut'.
彼は仕事で横着をして、上司に叱られた。
He cut corners at work and got scolded by his boss.
横着な態度を取ると、周りからの信頼を失うよ。
If you take a lazy attitude, you'll lose the trust of those around you.
na-adjective, noun
impudent; brazenly dishonest; shameless
A stronger, more negative sense: acting with audacity or dishonesty, often in a way that disregards others. Less common than the laziness sense, but still used in modern Japanese.
あの政治家の横着な振る舞いには呆れるばかりだ。
I'm simply disgusted by that politician's brazen behavior.
横着にも彼は自分のミスを認めようとしなかった。
Shamelessly, he refused to admit his own mistake.