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source (n.)

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, arise, get up, mount up, ascend; attack,“ a contraction of surrigere, from assimilated form of sub „up from below“ (see sub-) + regere „to keep straight, guide“ (from PIE root *reg- „move in a straight line,“ with derivatives meaning „to direct in a straight line,“ thus „to lead, rule“).

The meaning „a first cause, an origin“ is from late 14c., as is that of „fountain-head of a river.“ The meaning „person or written work supplying information or evidence“ is by 1777.

from: https://www.etymonline.com/word/source