i-adjective
Describes someone acting overly familiar, cocky, or disrespectful, often with a nuance of being presumptuous or too big for one's britches. Colloquial and somewhat old-fashioned.
あの新人はしゃらくさい口をきく。
That new guy talks cheekily.
しゃらくさいことを言うな。
Don't be impertinent.