marinade: (noun) an acidic liquid, typically made of vinegar or wine with spices, used for soaking food to flavour or tenderize it
marinate: (verb) to soak something in a marinade
Language changes based on use. Dictionaries record how people use language even if the use is considered incorrect when it starts. Then once the dictionary includes the change, people think it must be right. Merriam-Webster now includes marinade as a verb, but if you look for the meaning, it will redirect you to marinate. Marinate has been in use since the 1640s and is still the preferred form for the verb.