From "invent" and "invitation". Lacking a formal invitation, inventing your own; to invite one's self
I knew they wouldn't mind me coming by, but they hadn't invited me; so I went on my own inventation.
That momentary feeling of accomplishment and success when you type in a difficult word and see that spell-check did not have correct it.
I just used the word ventriloquism in an email, and had a little moment of spellabration when I got it without spell-check having to correct it for me.