Know how to use "fewer" and "less"? Find out.
unable to be appeased; irreconcilable
Latin im- + placare 'to appease'
early 15c., from Old French implacable, from Latin implacabilis "unappeasable," from assimilated form of in- "not, opposite of" (see in- (1)) + placabilis "easily appeased" (see placate). Related: Implacably.