01 a. Not in the regular, usual, or established form; not according to official, conventional, prescribed, or customary forms or rules; irregular…
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1.
Not in the regular, usual, or established form; not according to official, conventional, prescribed, or customary forms or rules; irregular; hence, without ceremony; as, an informal writing, proceeding, or visit.
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2.
Deranged in mind; out of one's senses.[Obs.]“These poor informal women.” — Shak.