insoluble – a substance that can NOT be dissolved when mixed with a liquid

To remember the meaning of insoluble, use the mnemonic on the word soluble.

This will help you remember that soluble is when a substance CAN BE dissolved. 

The main difficulty lies in remembering what the word IN means. 

"IN" means "NOT" and comes from Latin. 

To remember that IN means NOT use the following mnemonic. 

The Inn (in) looked like a giant knot (not).

So now you should be able to remember that insoluble means not soluble or not able to be dissolved.