IPA: /ˈfiːblɪst/
KK: /ˈfibəlɪst/
The weakest or least strong; lacking in physical strength or force.
Despite his feeblest attempts to lift the heavy box, he couldn't move it at all.
Comparative: weaker
Superlative: weakest
Feeblest → It is formed from "feeble" (from Old French *feeble*, meaning weak or frail) and the suffix "-est" (meaning the most). The word "feeblest" means the most weak or frail.
Think of 'feeble' meaning weak, and remember that 'feeblest' refers to the most weak.