"Coherent" means logical and consistent, well-organized. The opposite would be "illogical," meaning not making sense, or lacking reason or clear structure.

Opposite words: Illegitimate, Weak, Absurd, Fatuous.

"Adulteration" refers to the act of adding impurities or inferior substances, usually to food or drink. The opposite would be "purification," meaning the process of removing impurities or making something pure.

Opposite words: Purify, Distill, Purge, Sanitize, Sterilize.

"Impromptu" refers to something done without preparation or rehearsal. The opposite would be "rehearsed," meaning something that has been prepared or practiced beforehand.

Opposite words: Prepared, Planned, Premeditated, Rehearsed.

"Divulge" means to reveal or make something known, especially information that was meant to be kept secret. The opposite would be "keep secret," which means to conceal or withhold information.

Opposite words: Befog, Camouflage, Conceal, Disguise, Eclipse.

"Indigenous" refers to something originating naturally in a particular place. The opposite would be "acquired," meaning something gained or obtained from an external source, not native or local.

Opposite words: Acquired, Imported, Exotic, Foreign, Alien, External, Overseas, Remote.

"Amalgamate" means to combine or merge things into one. The opposite of this would be "separate," which means to divide or keep things apart.

Opposite words: Cleave, Detach, Dissolve, Disperse, Scatter, Separate.

"Fastidious" describes someone who is very concerned about accuracy and detail, often hard to please. The opposite would be "easygoing," which refers to a person who is relaxed and unconcerned with details.

Opposite words: Relaxed, Affable, Lax, Breezy, Easygoing, Lenient.

"Castigate" means to criticize or reprimand severely. The opposite would be "extol," which means to praise highly or lavish praise upon someone or something.

Opposite words: Accolade, Eulogy, Acclamation, Laud, Extol.

"Attenuate" means to reduce the force, effect, or severity of something. The opposite of this would be "strengthen," which means to make something stronger or more intense.

Opposite words: Enhance, Inflate, Extend, Amplify, Escalate, Strengthen, Aggrandize.

"Alacrity" means cheerful readiness, promptness, or willingness to do something. The opposite would be "sluggishness," meaning a lack of energy, enthusiasm, or promptness.

Opposite words: Sluggishness, Idleness, Indolent, Lethargic, Torpid, Inertness.