It begins when a male goblin, wandering about on a mission of dubious nature, discovers a strange bush. A bush with legs. Green legs at that. Upon investigation he discovers a female goblin in the bush and has some fun.
Approximately two to three weeks later, as the female is walking through the woods, a baby goblin suddenly appears on the floor. Baby goblins do not cry. They are also immune to practically everything (magic, poison, disease, weapon blows etc) which is just as well, as goblins make appalling parents. After a week or two the mother gets bored of carting the baby around everywhere, and so leaves it behind in a bush.
When in the bush the baby sleeps very deeply for a couple of months (a sleep not dissimilar to that experienced by an old tired goblin), in which it loses its childhood invulnerabilities, and slowly gains its adult goblin abilities (generally the ability to wear armour and weild a sword and daggers comes first, followed by the slow seeping in of Corporeal into our beings). If a goblin is woken up prematurely it tends to be a bit strange (See Frik), however if it is left for the right amount of time it ends up as a normal goblin (Corp 2, sword, shield, light armour and two free points).
It is a commonly held belief amongst some peoples in the Jhereg that goblin babies are for playing rugby with. Very few goblins hold this belief, and find themselves confronted by a very angry goblin mother if rugby is attempted.