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Webster 1913 Edition

Adj.

[L. pref.

im-

not +

pacare

to quiet. See

Not to be appeased or quieted.

[Obs.]

Spenser.

–

Im-pa′ca-bly

adv.

Adj.

[L. in and paco, to appease.]

Not to be appeased or quieted.

impacable ‎(comparative more impacable, superlative most impacable)

(obsolete) Not to be appeased or quieted.
(Can we find and add a quotation of Spenser to this entry?)