If we think of words that have '-less' as a suffix, it is usually easy to fathom what you are without. For example, if you are 'tactless' you are without tact and if you are 'useless' you are without use, and so on. But what about 'gormless'?
'Gorm' comes from an old Scottish word 'gaum' meaning attention or notice. Therefore, someone who doesn't pay you or something enough attention or take adequate notice is described as 'gormless'.
And as an afterthought, the 'feck' in feckless has a similar story. If you are 'feckless', it means you may lack initiative or have a character flaw. Similarly, 'feck' derives from 'fek', again Scottish for 'effect'. 'Fek' to the Scots can mean 'value', 'return' and 'amount'. Simply, if you are accused of being 'feckless', it means you have been accused of being without worth or value.
In this light, 'feckless' has a definition that makes the word sound more of a stinging attack on someone than possibly intended.