gratuity
n.
1520s, "graciousness," from French gratuité (14c.) or directly from Medieval Latin gratuitatem (nominative gratuitas) "free gift," probably from Latin gratuitus "free, freely given" (see gratuitous). Meaning "money given for favor or services" is first attested 1530s.
〔李〕[grat=pleasing使人高兴的；-u-;-ity n.=things with the quality有……性质的东西→“things causing pleasure使人高兴的东西”→] a. gift of money given for some service;tip赏金；小费