(Anglo-Saxon). A graft; whence also a child; as, “You little imp.” In hawking, “to imp a feather” is to engraft or add a new feather for a broken one. The needles employed for the purpose were called “imping needles.” Lord Cromwell, writing to Henry VIII., speaks of “that noble imp your son.”
“Let us pray for ... the king's most excellent majesty and for ... his beloved son Edward, our prince, that most angelic imp.” —Pathway to Prayer.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894