prompter

late Middle English word dating back to 1400–50; see origin at prompt, -er¹

1300–50; (v.) Middle English < Medieval Latin prōmptāre to incite, Latin: to distribute, frequentative of prōmere to bring out, equivalent to prō- pro-¹ + ( e) mere to take, buy; (adj.) late Middle English < Latin promptus ready, prompt, special use of past participle of prōmere

Prompt, adjective and verb, presents some oddities. One is that the first recorded date for the adjective is about 1425 and for the verb, 1428, making it impossible to determine which part of speech was the source for the other. A second oddity is that prompting, the gerund (verbal noun) logically derived from prompt and meaning “incitement or impulse to action,” is first recorded in 1402, a quarter of a century before the verb. A third difficulty is that the Medieval Latin verb promptāre, the possible source of the English word, does not exist per se but is inferred from its Medieval Latin derivative noun promptātor “one who incites or urges,” recorded in the mid-15th century, and the Old Italian verb prontare “to urge, press.”
The commercial sense of the noun prompt “a time limit given for payment for merchandise purchased" dates from the mid-18th century. The computer sense of the noun “a message or symbol on a display screen requesting more information from a user" dates from 1977.