shortly

Old English already had this meaning: sċortlīċe meant "before long; soon," built from short and -ly just as the modern word is. Today it most often pins an event close to a fixed moment, as in shortly after or shortly before (shortly after the 2016 election). On its own it points forward, as when a servant in the American novelist Winston Churchill's The Celebrity (1897) brings word that his master "would shortly join" the narrator.

Here the shortness is in the telling, not the timing. Fielding's narrator in Tom Jones (1749) feels obliged "shortly to hint a matter" he would rather spare, meaning he will mention it in few words. Because "she will explain shortly" is easily heard as the "soon" meaning, briefly or concisely say the same thing without the risk of being misread.

The "short" behind this sense is the one in being short with someone: clipped, impatient, sparing with words. It is most at home after verbs of speaking (she said shortly, he answered shortly), where fiction uses it to mark a tone without spelling it out. Curtly is its closest match, while abruptly puts more weight on suddenness than on temper.