ailing is the present participle of ail, from Middle English ailen, from Old English eglan ('to trouble, afflict'), from egle ('troublesome, painful'). The path is chiefly one of transmission: an older form is taken into English, with the word's shape and meaning carried through the language names and forms preserved in the history. In modern use, the adjective sense is "in poor health or in difficulty; unwell or failing", which keeps the etymology connected to the word's present dictionary value rather than treating it as a merely historical label. The wider word family includes related English forms such as ail, ailment, showing how the same base continues to support neighboring English vocabulary. The principal historical forms in the chain include Old English eglan, Proto-Germanic *aglijaną, which anchor the explanation in named linguistic material.