IPA: /ɪnˈsɪstə/
KK: /ɪnˈsɪstər/
A person who firmly demands something or expresses a strong opinion.
She is an insister on the importance of education for all children.
Insister is formed from "in-" (meaning in or upon) and "sistere" (meaning to stand). The word means to stand firm or persist in a position or argument.
Think of standing 'in' a position ('in-') and 'standing firm' ('sistere') — that's why insister means to persist or stand firm in an argument.