can I use function generator as electrocutaneous
Various TENS designs use relatively high voltage (30 to 100 volts), so you will need to somehow step up the voltage and then regulate the current, as Arduino can only give 5 volts and cannot regulate the current. You can build this with Arduino, but you will have to design it very carefully, otherwise you may deliver a dangerous voltage instead.

'function generator' is a common term for all circuits which generate impulses. While some function generators may generate what you need, most generators only output up to 10 volts and cannot regulate the current.