arrear

To drive or herd animals along (arrear el ganado, to drive the cattle) and, colloquially, to give a sudden blow (arrear un golpe/una torta, to land a smack). The imperative ¡arrea! also survives as a common Spanish interjection of surprise.

• El pastor arreaba las ovejas de vuelta al corral. — The shepherd was driving the sheep back to the pen.
• Le arreó un golpe al ver que le robaba la cartera. — He landed a blow on him when he saw him stealing his wallet.
• ¡Arrea, qué caro está esto! — Wow, this is expensive!

Missing that ¡arrea! functions as a standalone interjection of surprise in Spain, unrelated to any literal driving of animals — a learner who only knows the herding sense will be confused hearing it used that way in conversation.

• expression ¡Arrea! — Common Spanish interjection of surprise or admiration ("¡arrea, vaya gentío!"), derived from the imperative of arrear via the related herding cry ¡arre!, documented in Spanish etymological dictionaries.