Shaykh Ahmed Zarruq (Allah have mercy on him) said,

‘The beginning of each sin is a random thought (khatrah).

It should be fought with loathing (karahah).

Otherwise it becomes a (recurring) opposition (mu’aradah). Here it should be met with aversion otherwise it becomes a (repeated) whispering (waswasa). Here it should be resisted with great effort (mujahida). Otherwise it becomes lust (shahwah) accompanied by capricious desire (hawa). Here all reason, knowledge and understanding will be obliterated and he will become to weak to save himself from sinking in to the bottomless pit of sin and crime.’

Ahmad Zarruq, p291