Webster's Revised Unabridged Dictionary · 1913

Informal

Informal , adjective

[Prefix in- not + formal.]

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.
2.
Deranged in mind; out of one's senses. [Obsolete]
These poor informal women. — Shakespeare