"Solvent" refers to a financial state where someone has enough assets to cover their liabilities or debts. The opposite would be "precipitant," which in this context refers to someone who is financially unstable or unable to meet their obligations, often due to liabilities exceeding assets.

Opposite words: Waning, Worthless, Precipitant, Creditor.

"Obdurate" refers to someone who is stubborn and unwilling to change their opinion or course of action. The opposite would be "complaisant," which means willing to please others or inclined to be agreeable and accommodating.

Opposite Words: Merciful, Gracious, Benignant, Bend, Brittle. 

"Caustic" refers to something that is harmful, corrosive, or capable of burning or eroding. The opposite would be "innocuous," meaning harmless or not likely to cause injury or damage.

Opposite words: Amusing, Mild, Playful, Innocuous, Bland, Suave.

"Rarefy" means to make something thinner, less dense, or more diffuse. The opposite of rarefy would be "condense," which means to make something denser or more concentrated.

Opposite words: Solid, Heavy, Condense.

To "convict" someone is to declare them guilty of a crime, often following a trial. The opposite of this is "acquit," which means to declare someone not guilty or free from criminal charges.

Opposite words: Acquit, Clear, Absolve, Release, Exempt.

The word meteoric means sudden and brilliant, often referring to a rapid rise. Its antonym is gradual, which means slow or progressive over time.
Opposite words: Gradual, Slow, Steady, Prolonged, Deliberate.

The word substantive means important or meaningful. Its antonym is trivial, which means insignificant or unimportant.
Opposite words: Trivial, Insignificant, Minor, Superficial, Unimportant.

The word abstinence means self-restraint or refraining from indulgence. Its antonym is indulgence, which means yielding to desires or pleasures.
Opposite words: Indulgence, Excess, Gluttony, Enjoyment, Gratification.

The word feral means wild or untamed, often used for animals. Its antonym is tame, which means domesticated or controlled.
Opposite words: Tame, Domesticated, Controlled, Civilized, Trained.

The word relinquish means to give up or surrender. Its antonym is possess, which means to own or keep.
Opposite words: Possess, Retain, Hold, Keep, Maintain.