From The Collaborative International Dictionary of English v.0.48:
groat \groat\ (gr[add]t or gr[=o]t), n. [LG. gr[=o]te, orig.,
great, that is, a great piece of coin, larger than other
coins in former use. See Great.]
1. An old English silver coin, equal to four pence.
[1913 Webster]
2. Any small sum of money.
[1913 Webster]