57Gregy
If it didn't half rain, does that mean it did half rain? In other words, it rained?
It didn't half rain, it whole rained.
Simples.
It's a way of saying there was no half-measure or restraint involved. A bit like you guys would possibly say "It rained, and then some".
e.g. "That Jonbouy, he doesn't half go on".
Of course that statement is very different from the truth but it serves an example...