mid-14c., "support, base," from Old French sourse "a rising, beginning, fountainhead of a river or stream" (12c.), fem. noun taken from past participle of sourdre "to rise, spring up," from Latin surgere "to rise" (see surge (n.)). Meaning "a first cause" is from late 14c., as is that of "fountain-head of a river." Meaning "written work (later also a person) supplying information or evidence" is from 1788.