IPA: /ˈtjuːn.ləs.li/
KK: /ˈtunləsli/
In a way that lacks melody or harmony; without a tune.
She sang tunelessly, making everyone laugh at her off-key performance.
Tunelessly → It is formed from "tune" (from Old French *ton*, meaning a musical sound or melody) and "-less" (meaning without) and "-ly" (meaning in a manner). The word "tunelessly" means in a manner that is without melody or musical quality.
Think of 'tune' as a musical sound and remember that 'tunelessly' means without a tune, or lacking melody.