is it safe to fully discharge the 1.2v 2100ma nimh AA batteries using 5 ohms resistor, just like simulating a 240ma load.

Make sure your resistor is at least 1 watt or more since it will heat up eventualy the resistance value will drop, otherwise you'll kill the resistor.

If your 5 ohms resistor looks rectangular, white and as big as you 1.2 battery. You're more than OK.

Yes it should be safe.

1.2V / 5R = 240mA, but as the voltage drops, so does the current, so at 1V, the current draw is 200mA and so on.

Assuming 240mA X 1.2V = 0.288W, so a 1W resistor or higher is suitable (1W = extra large version of normal resistor, or use the rectangular white 5/10W versions).

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BTW rexter, inside standard resistors, resistance actually INCREASES with heat, so there is a self regulating effect there.

(Heat goes up, so resistance increases, and current draw decreases. Because current decreases, it cools down and resistance falls, allowing more current to flow again. etc etc)