"happening or being every day," mid-15c.; see day + -ly (1). Compare Old English dæglic, a form found in compounds: twadæglic "happening once in two days," þreodæglic "happening once in three days." The more usual Old English adjective was dæghwamlic (also dægehwelc), which became Middle Englishdaiwhamlich. Cognate with German täglich.
As an adverb, "every day, day by day," early 15c. (the Old English adverb was dæghwamlice. As a noun, "a daily newspaper," by 1832.