pronto adv.

mid-14c., prompten, "to incite to action, urge," from the adjective or from Latin promptus, past participle of promere "to bring forth," from pro "forward" (from PIE root *per- (1) "forward") + emere "to take" (from PIE root *em- "to take, distribute").

The meaning "coach (someone), assist (a learner or speaker) by suggesting something forgotten or imperfectly learned or known" is from early 15c.; specifically in the theatrical sense of "to assist a speaker with lines" by 1670s. Related: Prompted; prompting.

Proto-Indo-European root meaning "to take, distribute." 

It forms all or part of: assume; consume; emption; example; exemplar; exemplary; exemplify; exempt; exemption; impromptu; peremptory; pre-emption; premium; presume; presumption; prompt; pronto; ransom; redeem; redemption; resume; sample; sejm; subsume; sumptuary; sumptuous; vintage.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit yamati "holds, subdues;" Latin emere "buy," originally "take," sumere "to take, obtain, buy;" Old Church Slavonic imo "to take;" Lithuanian imu, imti "to take."

For the sense shift from "take" to "buy" in the Latin verbs, compare Old English sellan "to give," source of Modern English sell "to give in exchange for money;" Hebrew laqah "he bought," originally "he took;" and colloquial English I'll take it for "I'll buy it."