@larsman Spend a penny means to go to the toilet, especially a public toilet. One usually is said to be going to spend a penny. The expression is derived from the fact that public toilets were installed in the United Kingdom in the mid-1800s that required a penny to be unlocked.
On Bob Dylan's Tempest LP, song Roll On John, he sings the line "Another bottle's empty, another penny spent". A little tip of the hat to Lennon's home country. 😁

