The term INCREDIBLY SILLY can be broken down into two crossword solutions: INANE (5 letters) and SILLY (5 letters). INANE is an adjective meaning 'lacking sense or substance; foolish'. It is derived from the Latin word 'inanis', which means 'EMPTY'. SILLY is an adjective meaning 'foolish, stupid, or lacking good sense'. It is derived from the Old English word 'seolh', which means 'weak, foolish, or SILLY'. Together, these two words describe something that is incredibly foolish and lacking in good sense.