1520s, "to give (legal) confirmation to," from Latin corroboratus, past participle of corroborare "to strengthen, invigorate," from assimilated form of com "with, together," here perhaps "thoroughly" (see com-) + roborare "to make strong," from robur, robus "strength," (see robust).
Meaning "to strengthen by evidence, to confirm" is from 1706. Sometimes 16c.-18c. in its literal Latin sense "make strong or add strength to," especially of medicines. Related: Corroborated; corroborating; corroborative.
It forms all or part of: bilirubin; corroborate; Eritrea; erysipelas; erythema; erythro-; Radnor; red; redskin; roan; robust; rooibos; Rotwelsch; rouge; roux; rowan; rubella; rubicund; rubric; ruby; ruddock; ruddy; rufous; Rufus; russet; rust.
It is the hypothetical source of/evidence for its existence is provided by: Latin ruber, also dialectal rufus "light red," mostly of hair; Greek erythros; Sanskrit rudhira-; Avestan raoidita-; Old Church Slavonic rudru, Polish rumiany, Russian rumjanyj "flushed, red," of complexions, etc.; Lithuanian raudas; Old Irish ruad, Welsh rhudd, Breton ruz "red."
Middle English affermen, affirmen, "to decide upon" (c. 1300); "to state positively" (late 14c.), from Old French afermer (Modern French affirmer) "affirm, confirm; strengthen, consolidate," from Latin affirmare "to make steady, strengthen," figuratively "confirm, corroborate," from ad "to" (see ad-) + firmare "strengthen, make firm," from firmus "strong" (from suffixed form of PIE root *dher- "to hold firmly, support").
The spelling was refashioned 16c. in French and English on Latin model. Legal sense "declare solemnly (as before a court) but without an oath" is from early 15c. Related: Affirmed; affirming.