Informal1. (a.) 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. (a.) Deranged in mind; out of one's senses.
Thesaurus Entries
Bohemian, affable, beat, beat-up, bedraggled, blowzy, breakaway, careless, casual, chintzy, colloquial, common, conversational, cordial, degage, dilapidated, drabbletailed, draggled, draggletailed, easy, easygoing, en deshabille, everyday, familiar, far out, folksy, free, free and easy, fringy, frowzy, frumpish, frumpy, garden-variety, gracious, grubby, haymish, heretical, heterodox, hippie, homely, homey, in dishabille, in rags, irregular, kinky, loose, lumpen, maverick, messy, mussy, natural, negligent, nonstandard, not cricket, not done, not kosher, offbeat, offhand, offhanded, ordinary, original, plain, poky, private, ragged, raggedy, relaxed, ruinous, scraggly, seedy, shabby, shoddy, simple, slack, slatternly, slipshod, sloppy, slovenly, sluttish, sociable, sordid, special, spoken, spontaneous, sporty, squalid, substandard, tacky, tattered, unaffected, unassuming, unauthorized, unceremonious, unconstrained, unconventional, uneducated, unfashionable, unkempt, unliterary, unneat, unofficial, unorthodox, unpretentious, unsightly, unstudied, untidy, vernacular, way out