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Webster 1913 Edition

(ĭm-pērs′ȧ-b’l)

Adj.

Not capable of being pierced; impenetrable.

[Obs.]

Spenser.

Adj.

impers'able. [in and pierce.]

Not to be pierced or penetrated.

impierceable ‎(comparative more impierceable, superlative most impierceable)

(obsolete) Not capable of being pierced; impenetrable.
(Can we find and add a quotation of Spenser to this entry?)