IPA: /ˈtraɪflər/
KK: /ˈtraɪflər/
A person who treats something as unimportant or trivial, often engaging in superficial or careless behavior.
He was seen as a trifler, never taking his responsibilities seriously.
Trifler → It is formed from "trifle" (from Old French *trufle*, meaning something of little value or importance) and "-er" (meaning a person who). A trifler is a person who engages in trivial matters or treats things as unimportant.
Think of someone who deals with things of 'little value' ('trifle') — that's why a trifler is someone who treats matters lightly.