A Quaalude, Sopor, or Parest (methaqualone) tablet. It was coined in the mid-70's, before most people ever heard of Ecstasy. It is a hypnotic (sleeping pill) that was popular due to its side-effect of lowering sexual inhibition in females (it tended to make males somewhat impotent.) The term disco-biscuit (with a hyphen) shifted to refer to Ecstasy (and the hyphen was dropped) when methaqualone was made a Schedule I drug and withdrawn from the US market in 1984.
Give that girl a disco-biscuit and you're sure to get laid.