Any time you like - though I would advise against it when walking through a shopping mall! I use the word in preference to 'Eureka!' when I find something that's been mislaid.
If I cut myself, I change it to 'Germoline'!
'Geronimo' was a corruption of 'Jerome', which was the name given to him by the Mexicans, who he fought against.
His actual Apache name was 'Goyathlay' which means 'one who yawns'.
I think he was a North American india chief and members of the Airborne Regiment shouted out his name whenever they jumped out of their planes over enemy territory during the WW2 years but I don't know what the connection was, probably it had something to do with America Parachutest's