(slang) to describe someone is who is absentminded, scatterbrained and lost in thought; an inattentive person
Definitions (粵典–words.hk)
名詞
失魂嘅人；唔小心，成日唔記得呢樣唔記得嗰樣、唔見呢樣唔見嗰樣嘅人（量詞：條／隻） a careless and forgetful person; literally 'a fish which has lost its soul'