Least intrusive
As human beings, when we face situations where our resources are being challenged, we feel stressed. There are situation, where in spite of being stressed, situation demands us to perform, however, during those times, we are often not at in a state to state to invest our complete output, because the stressful thoughts keep coming into our thought process, obstructing our proper functionality. The thoughts that bars us from performing at our optimum efficiency is termed as intrusive thoughts.
We often call a thought least intrusive, when it reappears but at a very low frequency, although it might not be appearing all the time, it does cause some distress in the individual. For instance, when a person suffers from obsessive compulsive disorder, the unwanted, stressful and repeatative intrusive thoughts keeps on reappearing persistently to an extent that the person is motivated to perform compulsive behaviour in order to get away with those thoughts. This sometimes effects the personal and occupational functionality of the person, leading to severe maladjustment and distress.