to certain extent or degree
"pretty big"
"pretty bad"
"jolly decent of him"
"the shoes are priced reasonably"
"he is fairly clever with computers"
with good sense or in a reasonable or intelligent manner
"he acted sensibly in the crisis"
"speak more sanely about these affairs"
"acted quite reasonably"
In accordance with reason.
"The committee reached a reasonably balanced conclusion after reviewing all the evidence."
In plain English: Reasonably means doing something with good judgment and fairness, not going too far but also not being lazy.
"The prices for these shoes are reasonably low compared to other stores."
This word comes from Middle English resonably, which was formed by adding the suffix -ly to reasonable. It entered English with a meaning very similar to its current usage of "in a fair or sensible manner."