fairly (adv.) Look up fairly at Dictionary.com
c. 1400, "handsomely," from fair (adj.) + -ly (2). Meaning "impartially, justly" is from 1670s. Sense of "somewhat" is from 1805, a curious contrast to the earlier, but still active, sense of "totally" (1590s). Old English had fægerlice "splendidly."