As far as I can read from this answer _italiana/domande_e_risposte/lessico/lessico_371.html the usage of "prego" started to spread from the XIX century and derived from the german "bitte"; it started to be used in the italian regions under the area of influence of the Austro-Hungaric Empire (that means northen Italy) among the members of the high society and means "prego l'onore del vostro comando" that is "I pray/seek/request the honor of your command" ("I am at your full disposal")
Acquerello italianoIT 1.01760-minute audiocassettes containing songs, interviews, and cultural information about Italy; a tapescript and a study guide accompany each cassette; study supplement contains listening comprehension exercises, crossword puzzles, and an answer key.
prego italian textbook 6th edition answer key 15
2ff7e9595c