IPA: /ˈlaɪflɛsli/
KK: /ˈlaɪfˌlɛsli/
In a manner that shows no energy, spirit, or vitality; appearing dull or inactive.
The actor delivered his lines lifelessly, failing to engage the audience.
Lifelessly → It is formed from "life" (from Old English "līf", meaning existence or living) and "-less" (meaning without) and "-ly" (meaning in a manner). The word "lifelessly" means in a manner that is without life or vitality.
Think of 'life' being absent ('-less') and how it is done 'in a manner' ('-ly'). This helps you remember that 'lifelessly' describes doing something without any life or energy.