means To produce offspring or new things in great numbers — literally the release of eggs by fish and amphibians, and figuratively the bringing-forth of anything that proliferates.

from From Anglo-Norman espaundre and Old French espandre, 'to spread out, pour forth,' which traces back to Latin expandere, 'to spread, stretch out' (the same root that gives us 'expand'). The fishy sense came first in English — the scattered, outpoured eggs of fish and frogs — and the image of something spreading and multiplying did the rest.