a coined variation of the expression
shoot one's wad, meaning, to say everything on one's
mind about a
particular subject. To spill something means to divulge or make known and one's wad here implies the entire thing, all one has. The expression
shoot one's wad comes from the activity of gambling, where it is said that if one uses up his entire
resources, he has
shot his wad; the wad being his bankroll or
money.