"happening or being every day," mid-15c.; see day + -ly (1). Compare Old English dglic, a form found in compounds: twadglic "happening once in two days," reodglic "happening once in three days." The more usual Old English adjective was dghwamlic (also dgehwelc), which became Middle English daiwhamlich. Cognate with German tglich.
As an adverb, "every day, day by day," early 15c. (the Old English adverb was dghwamlice. As a noun, "a daily newspaper," by 1832.