There are 4 morphemes in inconceivable. 'Conceive' consists of two, 'con', which means either 'with', 'together' or 'thoroughly', as seen in words such as connect, 'join with' or conjecture, 'thrown together'. 'Ceive' generally means 'get', such as in the word 'transceive', to get across. Cobble them together and you get 'get with'. So if something is inconceivable, your brain can't get with, or understand it.