Chicago--Jan. 4--Lackluster December sales at U.S. retailing chains were a sharp disappointment after the boom years of the late 1990s. Shoppers felt the pinch of the slowing economy, higher energy bills and sagging stock market portfolios, resulting in cautious buying even with heavy promotions and markdowns. December chain store sales posted an aggregate gain of 0.7% from 1999 levels, according to the Bank of Tokyo-Mitsubishi/UBS Warburg index of 81 retailers. It marked the worst December since at least 1986 and the worst overall holiday season since 1995.