Definition of jigger:
(n) :
a double-ended vessel, generally of stainless steel or other metal, one end of which typically measures 1 1/2
fluid ounces, the other typically 1 fluid ounce; one who jigs
(v) :
to alter or adjust, particularly in ways not originally intended; to use a jigger
"you'll have to jigger it from the original specifications to get it to work."