Mary: 
 
The OED entry on coercion includes to govern by force as the second 
definition of the term, and cites as popular usage names like "The Coercion 
Act".  It dates this standard association of government & force to late 
18c.  Under coerce, the usage is pushed back to Butler in the mid 17th 
century, though there it seems to be being used with a tone of 
approval--not as the evil constraint on liberty so much as the necessary 
constraint of the licentious.  The OED datings are, I think, increasingly 
regarded as suspect (indeed, there was a good deal of controversy about the 
lack of revision in OED2), and so you could probably assume that the usage 
is even older.  Somewher, EPThompson comments with cryptic humour re the 
OED that the first cuckoo often sang before the fact was recorded in the 
Times.  Either way, to address your question, the usage probably predates 
economists.  Though the shift from the (17c) right to constrain the errant 
to the (19c) suspect restraint of "natural" freedoms presumably accompanied 
the related economic debate.  Of course, the term was used only to 
distinguish some forms/acts of governance from others, but once you believe 
that all government is the restraint of liberty by force, then govern and 
coerce become synonymous. 
 
Best wishes, 
Paul 
 
Paul Duguid 
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