1560s, from un- (1) "not" + appeasable. Related: Unappeasably.
Desolate winds that cry over the wandering sea;
Desolate winds that hover in the flaming West;
Desolate winds that beat the doors of Heaven, and beat
The doors of Hell and blow there many a whimpering ghost;
O heart the winds have shaken, the unappeasable host
Is comelier than candles at Mother Mary's feet.
[W.B. Yeats, "The Unappeasable Host," 1899]