Synonyms for antediluvian

Most common

1. Archaic — of attitudes, ideas, etc., extremely old-fashioned, especially to a laughable extent; antiquated

2. Ancient — of a person or thing, very old; ancient

3. Preflood — belonging or pertaining to, or existing in, the time prior to the great flood described in Genesis, or (by…

adj. 1
Belonging or pertaining to, or existing in, the time prior to the great flood described in Genesis, or (by extension) to a great or destructive flood or deluge described in other mythologies.
adj. 2
Of a person or thing: very old; ancient.
adj. 3
Of attitudes, ideas, etc.: extremely old-fashioned, especially to a laughable extent; antiquated.
Those ideas are antediluvian.

Use in printed books, per decade from 1850 to 2019.

3 letters: old¹
4 letters: aged¹
5 letters: stale¹, olden², moldy², hoary²
6 letters: mouldy²
7 letters: ancient², antique², extinct², archaic³, defunct², disused, outworn
8 letters: backward², outdated³, obsolete³, decrepit³, outmoded³, preflood
9 letters: primitive³, venerable⁴
10 letters: antiquated⁴, fossilized³, fossilised
11 letters: prediluvian
12 letters: antediluvial
13 letters: superannuated

The small number after each word is its number of syllables (olden² = 2 syllables).