In Plautus's "Miles Gloriosus" Act 4; Scene 6:
ACROTELEUTIUM: "video. edepol nunc nos tempus est malas peioris fieri." =
ACROTELEUTIUM: "I see him. Troth, now is the time, in our mischief, for us to become supremely mischievous." (Perseus)
Literally: "....now is time (for) us to become bad things ("fieri malas") of the worse thing ("peioris")", which does not make much sense.
How does the genitive, "peioris", come to mean, "in our mischief"?
In this case, peiorīs has a long ī and is accusative plural. Once your realize that, the sentence becomes much easier to parse:
Nunc tempus est
nos malas
peioris fieri
=
Now is the time
for us, [who are already] bad,
to become worse.
"-is" and "-es" can both be used for the accusative plural of 3rd declension nouns and adjectives: see, for instance, Allen & Greenough §71 (nouns) and Allen & Greenough §117 (adjectives). For I-stem nouns and adjectives, "-is" is actually listed as the primary ending; for comparative adjectives, "-is" is listed as the secondary ending.
You can see a similar usage in Vergil:
Extemplo Aeneae solvuntur frigore membra:
ingemit, et duplicis tendens ad sidera palmas
talia voce refert: (Aeneid I.92-94)
or, in the famous speech by Turnus:
audentis Fortuna iuvat (X.284)
Anecdotally, I've noticed this form of the accusative plural a lot more often in Plautus and poets.
