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Webster 1913 Edition

Noun.

The act of impregnating, or the state of being impregnated, in addition to a prior impregnation; superfetation.

Noun.

[super and impregnation.] The act of impregnating upon a prior impregnation; impregnation when previously impregnated.

superimpregnation ‎(usually uncountable, plural superimpregnations)