With apologies to readers who might already know, Indefinite Hyperbolic Numerals* (IHNs) are words like zillion, jillion, and umpteen. Or, to be exact :
“Indefinite hyperbolic numerals (IHN) are words that (1) resemble numerals morphologically, and (2) act as numerals morphosyntactically within numeral phrases, yet (3) whose direct numerical referent remains indefinite.”
For an in-depth look at them, turn to the work of Professor Stephen Chrisomalis of Wayne State University, US. His paper : ‘Umpteen reflections on indefinite hyperbolic numerals’ appears in the journal American Speech, 2016 volume 91, number 1: 3-33.
In which, as part of his investigation, the author has compiled a new table of what he determines to be the earliest origins of IHNs. For example, both squillion and umpteen are found to have appeared as early as 1878, whereas umptillion only surfaced in 1948.
*Note: Some call them ‘Numericals’.