1. (v.t.)negate to deny the existence, evidence, or truth of (something).
2. negate to cause to be ineffective; nullify or invalidate (something).
3. (v.i.)negate to be negative; bring or cause negative results: a pessimism that always negates.
Etymology: (1615–25; < L negātus, ptp. of negāre to deny, refuse, der. of neg-, akin to nec not (cf. neglect ); see -ate1)
Definition of 'negate'
Princeton's WordNet
1. (verb)contradict, belie, negate be in contradiction with
2. (verb)contradict, negate, contravene deny the truth of
3. (verb)negate, contradict prove negative; show to be false
4. (verb)neutralize, neutralise, nullify, negate make ineffective by counterbalancing the effect of "Her optimism neutralizes his gloom"; "This action will negate the effect of my efforts"
Definitions of negate 
