1560s, "a driving or impelling thrust," from push (v.). By 1590s as "a vigorous attempt." By 1803 as "a determined advance, a pushing forward." The sense of "persevering enterprise, a determined effort to get on" especially if inconsiderate of others is by 1855. Phrase when push comes to shove "when action must back up threats" is by 1936. An earlier Middle English noun push "a pustule, pimple, boil" probably is from pus by influence of push.