Both Uses of

colloquialism
in
Main Street, by Sinclair Lewis

His colloquialisms seemed to Carol no more lax than their habitual slang.†
Chpt 38 *

Now it was she who grunted "Huh!" and ignored him, and felt independent and masterful as she shot up out of bed, turned her back on him, fished a lone and petrified chocolate out of her glove-box in the top right-hand drawer of the bureau, gnawed at it, found that it had cocoanut filling, said "Damn!" wished that she had not said it, so that she might be superior to his colloquialism, and hurled the chocolate into the wastebasket, where it made an evil and mocking clatter among the debris of torn linen collars and toothpaste box.†
Chpt 14

Definition:

a verbal expression that sounds informal -- as is normally expected in casual conversation rather than in formal writing