1660s, "lines by which a figure is delineated," from out- + line (v.). Literally the outer or exterior line, but used freely for the principal or distinguishing lines. Meaning "rough draft in words" is from 1759.
1762, "to draw in outline, draw the exterior lines of, sketch the main features of," from outline (n.). Meaning "to describe in general terms" is from 1847. Related: Outlined; outlining.
sketch the outline of the book
outline his ideas