ex-gratia: as a favor : done by government when someone dies of train or public accident.Gratis=free but without that accident happening,just for ur gratification.Gratuity iis also free,for service provided =TIP.
g+rat+is => rat is free
gratis= grant + is , grant from govt. is free
read last three words "tis" as "tip " so tip always given freely
Great Ass....If an ass is available for free its great