Definition
1 : firmly established by long persistence
2 : confirmed in a habit : habitual
Examples
Since Ernie is an inveterate liar, we naturally didn't believe him when he told us he'd met the movie star.
"As an inveterate letter writer, I started sending email as soon as I could sign on with dial-up, and became impatient to connect via DSL." — Deborah Lee Luskin, The Rutland (Vermont) Herald, 25 Feb. 2016