Exercise 2 solution (sample answer)

Jizz is the overall impression of a bird: its shape, posture, flight style and size, with colour, voice, habitat and location. It is useful because experienced birders can often place a bird in a family or genus at a glance, before they have seen any detailed marks ("it had the jizz of a thrush").

The wartime-acronym explanation (General Impression of Size and Shape, from the air force) cannot be right because the word was already in print in 1922, before that war.