1520–30; < Latincorrōborātus past participle of corrōborāre to strengthen, equivalent to cor-cor- + rōbor(āre) to make strong (derivative of rōbor, rōbur oak (hence, strength); see robust) + -ātus-ate1
1530s, "to give (legal) confirmation to," from Latin corroboratus, past participle of corroborare "to strengthen, invigorate," from com- "together" or "thoroughly" (see com-) + roborare "to make strong," from robur, robus "strength," (see robust).
Meaning "to strengthen by evidence, to confirm" is from 1706. Sometimes in early use the word also has its literal Latin sense, especially of medicines. Related: Corroborated; corroborating; corroborative.