See also: Bras
Because of hygiene issues, most charities will not take undies unless they are unused and still in the original packaging.
Unless the undies are in excellent condition and suitable for reuse, they should be thrown out.
Underwear cannot be recycled. But they can be passed on for reuse through Cloop bins.
If the undies are NEW and never used, they may be welcome by some welfare initiatives.
IF and only if, the underwear is new - still in the package or with tags - then you can freecycle it.