Abbreviation for the Latin quaque die, once a day, a unit of frequency traditionally used in medical prescriptions. This notation is sometimes modified for a lesser frequency by imbedding a number of days in the middle, as inq.2d., every two days
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}