According to an on-line dictionary, fug is a warm, stuffy, or smoky atmosphere in a room. If you capitalize certain letters you get FuG, which is an abbreviation for Funk-Gerät radio set.
That radio set was part of the Lichtenstein radar (that) was among the earliest airborne radars available to the Luftwaffe in World War II and the first one used exclusively for air interception. Developed by Telefunken, it was available in at least four major revisions, called FuG 202 Lichtenstein B/C, FuG 212 Lichtenstein C-1, FuG 220 Lichtenstein SN-2 and the very rarely used FuG 228 Lichtenstein SN-3.