at once (usually modifies an undesirable occurrence)
"he promptly forgot the address"
In prompt manner; both soon and quickly.
"The manager asked us to submit the report promptly, so I finished it within an hour."
In plain English: Promptly means doing something right away without any delay.
"Please call me back promptly after you finish your meeting."
Usage: Use promptly to describe actions that happen immediately or without delay, such as replying to an email right away. It emphasizes speed more than the mere passage of time implied by words like "soon."
Promptly comes from combining the word prompt with the suffix -ly. The term entered English via French and originally meant to be ready or quick at doing something.