C Q D

from The Collaborative International Dictionary of English v.0.48

C Q D \C Q D\
   In radiotelegraphy, the letters signified by the code call
   formerly used (cf. S O S) by ships in distress, formed by
   combining the code call C Q (formerly used as a general call
   for all stations) with D for distress.
   [Webster 1913 Suppl.]