late 13c., "dry measure of one-quarter bushel," of unknown origin; perhaps connected with Old French pek, picot (13c.), also of unknown origin (Barnhart says these were borrowed from English). Chiefly of oats for horses; original sense may be "allowance" rather than a fixed measure, thus perhaps from peck (v.).
"act of pecking," 1610s, from peck (v.). It is attested earlier in thieves' slang (1560s) with a sense of "food, grub."