Ablution in Catholicism is the part of the Mass following the Communion where the priest performs a ceremonial washing of his hands.
I don't know about the US, but friends tell me that in the UK military, the 'ablutions' are the same as the latrines;
on a ship they are known as 'the heads'.
COD doesn't have an entry for ablute, only for ablution.
I've heard the word often enough, but always as a kind of humorous euphemism, like 'powder my nose' or 'spend a penny'.