adverb/a ˌuɲa de kaˈbaʝo/
riding (a horse) as fast as one can; like a bat out of hell
“[…]salir pitando para Portugal a uña de caballo.” — get out there and make for Portugal as fast as his horse's legs could carry him.
Synonyms: a matacaballo, con viento fresco, cagando hostias, cagando cerillas, cagando leches