Colloquial – adjective

• Characteristic of or appropriate to ordinary or familiar conversation rather than formal speech or writing; informal (Dictionary.com)
• Using conversational style (Merriam-Webster)
• (Of words and expressions) Informal and more suitable for use in speech than in writing (Cambridge)

“The noun colloquy was first used in English to refer to a conversation or dialogue, and when the adjective colloquial was formed from colloquy it had a similar focus. Over time, however, colloquial developed a more specific meaning related to language that is most suited to informal conversation – and it ultimately garnered an additional, disparaging implication of a style that seems too informal for a situation. Colloquy and colloquial trace back to the Latin verb colloqui, meaning ‘to converse.’ Colloqui in turn was formed by combining the prefix com- and loqui, ‘to speak.’” – Merriam-Webster

What Is the Purpose of Colloquialism in Literature? (MasterClass)

“Writers use colloquial expressions to add authenticity to their work. Colloquialisms can also help with:

• Dialogue. Recreating casual communication through colloquial dialogue can add realism to a story and its environment. In Irvine Welsh’s Trainspotting, for example, the colloquial street language of the characters reflect their lives on the fringe of Scottish society; an example is the phrase ‘git aulder,’ meaning ‘get older.’
• Setting. Colloquialisms can also help establish and support the time and place of a story. In Fannie Flagg’s Fried Green Tomatoes at the Whistle Stop Cafe, the characters speak in a folksy, colloquial tone that represents rural Alabama during the first half of the twentieth century.
• Characters. Colloquialisms can also help establish a character’s backstory, including age and socioeconomic background. The narrator of J.D. Salinger’s classic The Catcher in the Rye, 16-year-old Holden Caulfield, is educated but uses expressions like ‘can’tcha,’ ‘helluva time,’ and ‘dough.’ Salinger’s use of these rougher colloquialisms highlights Holden’s rebellious streak.”