IPA: /ˈwiːkli/
KK: /ˈwikli/
Not strong; lacking in physical or mental strength.
He felt weaklier than before after the long illness.
Comparative: weaker
Superlative: weakest
Weaklier → It is formed from "weak" (from Old English "wac", meaning not strong) and the suffix "-ly" (meaning in a manner of). The word "weaklier" means in a manner that is less strong or feeble.
Think of someone who is 'not strong' ('weak') doing something 'in a manner' ('-ly') that shows their lack of strength.