tubelight
Tubelight (noun) means a long tube-shaped lamp that gives off light using fluorescence, or a fitting that holds one or more of these tubes.
Etymology
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.