koreanwikiproject states that '인마' is short for '이놈아', and is usually used towards males (which would make sense as '놈' is an impolite way of saying 'man'). Is there a female equivalent, or is there only this form of the word?
I once thought about what would be the closest English word to '인마' in terms of its connotation and usage and the one I came up with is 'You, little bastard' when it is used between close friends.
'인마' could be an offensive word when it is used in argument. However, it could also be used among close friends and a male senior (in a higher position) can use it to his male junior. For example, when a sergeant in the army calls a private or a CEO or director of a company calls their subordinates, '인마' could be used in both friendly and unfriendly ways.
I personally heard '인마' used among females (it is very rare though) and whatever they meant by it, it didn't mean '이놈아' as the opposite term '이년아' exists.
I wouldn't say there is a female equivalent of '인마'. '이년아' is used when females engage in argument. But strangely enough, '이놈아' is not broadly used when males argue or fight. It is rather used more broadly by seniors when they address juniors both in friendly and offensive (belittling) tone.
축하한다, 인마. 정말 잘 했어. Congratulations, you little bastard. You did a great job.
야 인마. 왜 반말이야? Hey, you bastard. Why do you talk down to me?
I hope you see the clear difference in the examples.