Definition of side glass in US English:

side glass

noun

  • 1A side window of a vehicle, especially (in early use) of a carriage.

  • 2A piece of glass fitted into the arm of a pair of (usually tinted) spectacles or goggles to shade or protect the eye from the side. Usually in plural Now rare and historical.

Origin

Late 16th century (in an earlier sense). From side + glass.