I'm rather confused by your dictionary definition of 'live off'. It doesn't mean 'get money from another source' at all.
If you 'live off' something or someone, it means that you depend on that source, or that person, for your support or survival. For example, parents might complain if their lazy grown-up son chooses not to get a job, and instead stays at home 'living off his parents' i.e. depending on them financially.
If you 'live off the interest' of your investments, it means that these investments earn so much money for you that you don't need to work. Each month, the interest payments provide you with all the money that that you need to live.