More fully "red rizzer": a redcurrant bush, Ribes rubrum; a fruit of this plant. Frequently attributive, as "rizzer-berry", "rizzer-bush". Also: (in full "white rizzer") a whitecurrant fruit or bush; (in full "black rizzer") a blackcurrant fruit or bush.
Late 16th century; earliest use found in Accounts of the Lord High Treasurer of Scotland. Probably a variant of russel, perhaps with assimilation of liquids (r…l goes to r…r), or perhaps showing suffix substitution; perhaps compare also French rousseur redness, reddish colour.