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Definition:

The term "multiply" means to greatly increase in number. It can also mean to cause something to increase in amount, such as causing pain to multiply.

God told animals and human beings to "multiply" and fill the earth. This was a command to reproduce many more of their own kind.
Jesus made the bread and fish multiply in order to feed the 5,000 people. The amount of food kept increasing so that there was more than enough food to feed everyone.
Depending on the context, this term could also be translated as "increase" or "cause to increase" or "greatly increase in number" or "become greater in number" or "become more numerous."
The phrase "greatly multiply your pain" could also be translated as "cause your pain to become more severe" or "cause you to experience much more pain."
To "multiply horses" means to "greedily keep acquiring more horses" or to "get large numbers of horses."