Yasna wrote: சித்தி
Generally, ச is pronounced [sa] rather than [t͡ʃa] IME, although lower-caste people are apparently more likely to use [t͡ʃa], which is probably the older pronunciation. In other words, people probably pronounce this more like [ˈsit̪i]. This term might have come from an earlier form like ?சிற்றி [ˈsitri] or something, which I guess would literally be something like 'small woman'.
சித்தி FWIU doesn't really mean the same things 'aunt' does but rather specifically 'mother's younger sister', 'father's younger brother's wife', or even 'father's second wife'. Some Malayalees say [t͡ʃɪˈtəpɛn] for 'father's younger brother', though in my family, we instead say [uˈpaːpɛn], and that is what my older niece calls me (my younger niece was just born earlier this year). [t͡ʃɪˈtəpɛn] would literally mean 'small dad'. We also have [kɔˈt͡ʃəmma], which is in fact a more general term for 'aunt' or even 'auntie' but reserved for middle-class women (in fact, it can also mean 'middle-aged middle-class woman' in general) and literally means 'small mother', using the word we usually use in Malayalam for 'small', cognate with one of the words for 'shrimp' in Malayalam (and probably in Tamil as well).