Today it means: Synonym of fluorescent lamp (“a gas-discharge lamp that uses fluorescence to produce visible light, especially one consisting of a tube filled with mercury vapour which emits ultraviolet radiation when an electric current is passed through it, causing fluorescence when the radiation hits a phosphor coating on the interior of the tube”); also, a light fixture containing one or more fluorescent lamps. Full definition →
From tube + light. Sense 2 (“dim-witted person”) refers to the tendency of some fluorescent tubes to flicker as well as the length of time it takes for some to light up completely after being switched on, which is likened to a person whose knowledge is inconsistent or who takes time to understand things.
Etymology from Wiktionary, CC BY-SA 4.0.