late 12c., "emit or perceive an odor," not found in Old English, perhaps cognate with Middle Dutch smolen, Low German smelen "to smolder" (see smolder). OED says "no doubt of Old English origin, but not recorded, and not represented in any of the cognate languages."
Someone should revive smell-feast (n.) "one who scents out where free food is to be had" (1510s, "very common" c.1540-1700, OED) and smell-smock "licentious man" (c.1550-1900). To smell a rat "be suspicious" is from 1540s.