to certain extent or degree
"pretty big"
"pretty bad"
"jolly decent of him"
"the shoes are priced reasonably"
"he is fairly clever with computers"
without favoring one party, in a fair evenhanded manner
"deal fairly with one another"
In a fair manner; fair; not biased or skewed or favouring a certain party
"The judge ruled fairly, ensuring that neither side received preferential treatment during the trial."
In plain English: Fairly means doing something in a way that is just and honest without favoring anyone.
"The test was fairly easy for everyone to pass."
Usage: Use fairly to mean acting with justice and impartiality when describing how someone treats others. Do not confuse this usage with modifying an adjective to indicate a moderate degree, such as saying "fairly large."
Fairly comes from the Middle English word fayrly, which was formed by adding -ly to the adjective fair. The term entered modern usage with the same meaning it held in its earlier form.