mid-15c., falsifien, "to prove false," from Old French falsifier "to falsify, counterfeit" (15c.), from Late Latin falsificare "make false, corrupt," from Latin falsus "erroneous, mistaken" (see false). Meaning "to make false" is from c. 1500. Earlier verb was simply falsen (c. 1200). Related: Falsified; falsifying.
"act of making false, false representation; a showing to be false or erroneous," 1560s, from Late Latin falsificationem (nominative falsificatio), noun of action from past-participle stem of falsificare "to falsify" (see falsify).
Sense evolved in Latin from "refurbish," to "alter appearance of," to "falsify (especially by adding new material)." Middle English had interpolen (early 15c.) in a similar sense. Related: Interpolated; interpolating.
1590s, "to confer the degree of doctor on," from doctor (n.). Meaning "to treat as a doctor, administer medical treatment to" is from 1712; sense of "alter, disguise for the purpose of deception, falsify" is from 1774. Related: Doctored; doctoring.
c. 1500, "act of adulterating; state of being debased by mixture with something else," generally of inferior quality, from Latin adulterationem (nominative adulteratio) "an adulteration, sophistication," noun of action from past-participle stem of adulterare "corrupt, falsify; debauch; commit adultery," from ad "to" (see ad-) + alterare "to alter" (see alter), though Watkins explains it as ad alterum "(approaching) another (unlawfully)." Meaning "a result of adulterating" is from 1650s.