once in a blue moon

:  one time and no more

:  at any one time :  under any circumstances :  ever

:  at some indefinite time in the past :  formerly

:  one single time :  one time at least

:  that once was :  former

:  at the moment when :  as soon as

:  of the color whose hue is that of the clear sky :  of the color blue (see ²blue 1)

:  bluish

:  discolored by or as if by bruising

:  a color whose hue is that of the clear sky or that of the portion of the color spectrum lying between green and violet

:  a pigment or dye that colors blue

:  bluing

:  to make blue (see ¹blue)

:  to turn blue

:  the earth's natural satellite (see satellite 2a) that shines by the sun's reflected light, revolves about the earth from west to east in about 29¹⁄₂ days with reference to the sun or about 27¹⁄₃ days with reference to the stars, and has a diameter of 2160 miles (3475 kilometers), a mean distance from the earth of about 238,900 miles (384,400 kilometers), and a mass about one eightieth that of the earth

:  one complete moon cycle consisting of four phases (see ¹phase 1)

:  satellite

:  to spend in idle reverie :  dream

:  to expose one's naked buttocks to

:  to spend time in idle reverie :  behave abstractedly