IPA: /ɪnˈfɜːmli/
KK: /ɪnˈfɜrmli/
In a weak or unsteady way, often due to illness or age.
He walked infirmly, struggling to maintain his balance.
Infirmly → It is formed from "in-" (meaning not) and "firmus" (from Latin, meaning strong or stable) and the suffix "-ly" (meaning in a manner). The word "infirmly" means in a manner that is not strong or stable, often referring to weakness or frailty.
Think of 'not strong' ('in-' means not and 'firmus' means strong) to remember that 'infirmly' describes a weak or unstable manner.