Countless means too many to count. Not infinite — just past the point where anybody would try.
It is an exaggeration by design, and it lives in front of plural nouns: countless times, countless hours, countless people. Formal writing prefers innumerable and everyday speech prefers loads, but countless sits comfortably in between. It is worth using only where the number really is large, since it loses all its force from overuse. The shape of it is plain enough, being count with -less on the end.
1adjectivecountlessToo many to be counted.
“She had told the story countless times.”
Also accepted in games: innumerable
Middle English
counten
English
count
English, today
countless
From count + -less. count comes from Middle English counten. From Middle English counten, borrowed from Anglo-Norman conter, from Old French conter (“add up; tell a story”), from Latin computō (“to compute”). In this sense, displaced native tellan, whence Modern English tell. a doublet of compute. Compare typologically reckon, счита́ть, счита́ться; the semantic evolution to санах.
Etymology from Wiktionary, CC BY-SA