haymaker (n.)

mid-15c. as the name of an agricultural occupation, "one who cuts and dries grass" (hay-making is attested from c. 1400); 1910 in the sense of "very strong blow with the fist," from hay + agent noun of make; the punch probably so called for resemblance to the wide swinging stroke of a scythe. Haymaker punch attested from 1907.