[small piece, fragment] late 14c., scrappe, "piece of food remaining after a meal" (usually plural), from Old Norse skrap "scraps; trifles," from skrapa "to scrape, scratch, cut" (see scrape (v.)).
Hence, "any remnant or small, detached piece" (1580s), typically negative (not a scrap) or in reference to something written or printed. The dismissive term scrap of paper is attested by 1840, made infamous in 1914 by the German chancellor's comment when violating the treaty that guaranteed Belgian neutrality.
The meaning "remains of metal produced or collected after rolling or casting to be reworked" is from 1790. Scrap-iron is attested by 1794.
"a fight, struggle, tussle," 1846, possibly a dialectal variant of scrape (n.1) on the notion of "an abrasive encounter" [Century Dictionary]. Weekley and OED suggest obsolete colloquial scrap "scheme, villainy, vile intention" (1670s).
"to fight, brawl, box," 1867, colloquial, from scrap (n.2). Intransitive sense of "argue heatedly, quarrel" is by 1895. Related: Scrapped; scrapping.
"to make into scrap, consign to a scrap-heap, break up (machinery) into scrap-iron," 1883 (in reference to locomotives), from scrap (n.1). Related: Scrapped; scrapping; scrappable.
1874 (Hotten), "pugilist, fighter," agent noun from scrap (v.2). Later used generally of anyone or anything that puts up a fight.