How to make out 'Have it in for'?

Hi,

Can anyone explain for me the following phrase:?

I understand that ‘He has it in for me’ = ‘He dislikes me’.
What is the ‘it’ for? and why can the combination come to mean ‘dislike’?

Thank you very much in advance. haihao

“have it in for someone” is an idiom. Besides disliking you, the person may also try to cause you some trouble or harm.

“it” has no specific meaning that I know of; “it” is just part of the idiom, like “has” and “in” are.

“Have it in for” implies that the subject has a (take your pick):

grudge
bitterness
resentment

“it” IMO represents these feelings.