Yours, Etc.: Origins and Uses of 8 Sign-Offs
c.1200, "in a hard manner, with great exertion or effort," from Old English heardlic "stern, severe, harsh; bold, warlike" (see hard + -ly (2)). Hence "assuredly, certainly" (early 14c.). Main modern sense of "barely, just" (1540s) reverses this, via the intermediate meaning "not easily, with trouble" (early 15c.). Formerly with superficial negative (not hardly).