scotch (v.) Look up scotch at Dictionary.com
"stamp out, crush," 1825, earlier "make harmless for a time" (1798; a sense that derives from an uncertain reading of "Macbeth" III.ii.13), from scocchen "to cut, score, gash, make an incision" (early 15c.), of unknown origin, perhaps [Barnhart] from Anglo-French escocher, Old French cocher "to notch, nick," from coche "a notch, groove," perhaps from Latin coccum "berry of the scarlet oak," which appears notched, from Greek kokkos. Related: Scotched; scotching.
Scotch (adj.) Look up Scotch at Dictionary.com
"of Scotland," 1590s, contraction of Scottish. Disdained by the Scottish because of the many insulting and pejorative formations made from it by the English (such as Scotch greys "lice;" Scotch attorney, a Jamaica term from 1864 for strangler vines).

Scotch-Irish is from 1744 (adj.); 1789 (n.); more properly Scots-Irish (1966), from Scots (mid-14c.), the older adjective, which is from Scottis, the northern variant of Scottish. Scots (adj.) was used in Scottish until 18c., then Scotch became vernacular, but in mid-19c. there was a reaction against it. Scotch Tape was said to be so called because at first it had adhesive only on the edges (to make it easier to remove as a masking tape in car paint jobs), which was interpreted as a sign of cheapness on the part of the manufacturers.
scotch (n.1) Look up scotch at Dictionary.com
1778, elliptical for Scotch whisky. See Scotch (adj.).
scotch (n.2) Look up scotch at Dictionary.com
"incision, cut, score, gash," mid-15c., related to scotch (v.).