Giga-annum

From Jenny Everywhere Wiki

Madame Tarsa once claimed to have “giga-annums” of experience wrapping up Christmas presents. (PROSE: The Winter Quests)

Behind the scenes

Under the SI prefix scheme "giga-" refers to 109 times the base, or billions. "Annum," of course, means year (as in "annual"). Therefore "giga-annums" is just a pretentious way of saying billions of years.