If you give something unstintingly, you give a lot of it, freely, and you do not hold any back. It is an adverb, so it goes with a verb: you give, help, praise, share, or work unstintingly. Two ideas are inside it at once, and both matter. First, the amount is generous — not a little. Second, and this is the part students miss, the feeling is willing. Nobody had to ask twice. A rich man who pays a big fine is giving a lot, but not unstintingly. A tired friend who sits with you all night, again, is. The word is warm; it is almost always used to praise someone. You will meet it in speeches, in thank-you letters, in obituaries, in book acknowledgements — 'she gave unstintingly of her time'. Notice that odd little 'of' — 'give unstintingly of your time / your energy / yourself' is a set pattern, quite formal and rather beautiful. It is a written word more than a spoken one. In everyday talk a Taiwanese student would say 'she was so generous with her time' and sound completely natural. Save 'unstintingly' for when you are writing something that matters. The root verb 'stint' means to give less than you should, to be mean with something — so 'unstintingly' is really 'without holding back'.

毫不吝惜地、不遺餘力地。中文的「不遺餘力」最接近，因為它同樣含有「心甘情願、全力以赴」的意思，不只是「給很多」。講付出時間或關愛時也可說「毫無保留地」。