Webster 's 1913 dictionary defines "incuse" as "Cut or stamped in, or hollowed out by engraving."  The earliest coins did not have a reverse design, but instead had only an incuse mark left by the punch which was struck to force the metal into the obverse die.  Not long after these first coins a reverse design was engraved into the punch.  Those initial reverse designs, and for several centuries in some places, were struck on the reverse of the coin within the incuse (usually square) left by the punch. 

|Dictionary of Roman Coins|

The Dictionary of Roman Coins definition and the line drawing example show a condition today called "brockage".  One side of a brockage is stamped in, thus "incuse".

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