As a noun soil
is (uncountable) a mixture of sand and organic material, used to support plant growth or soil
can be (uncountable|euphemistic) faeces or urine etc when found on clothes or soil
can be a wet or marshy place in which a boar or other such game seeks refuge when hunted.
As a verb soil
is to make dirty or soil
can be to feed, as cattle or horses, in the barn or an enclosure, with fresh grass or green food cut for them, instead of sending them out to pasture; hence (such food having the effect of purging them), to purge by feeding on green food.
As a proper noun underground is
(uk|rail transport) the london underground.